Online Guitar Tuning

Standard guitar tuning E A D G B E makes a good compromise between uncomplicated fingering for many chords and the ability to play common scales without too much movement of the left hand.
The separation of the first E and second B string, as well as the separation between the third G, fourth D, fifth A and sixth E strings by a 5-semitone interval allows notes of the chromatic scale to be played with each of the left hand fingers controlling one of the first four frets.
The separation of the second B and third G string by a 4-semitone interval breaks the fingering pattern of the chromatic scale but it facilitates the playing of common chords and scales.

E|-----FIRST STRING
B|-----SECOND STRING
G|-----THIRD STRING
D|-----FOURTH STRING
A|-----FIFTH STRING
E|-----SIXTH STRING

First try the high E-string (first string). Try to match the sound of your E-string to the sound of the E-string of the tuner. Listen to it as much as you need to. Follow your instincts. If you think you have matched the sound on the tuner, you will probably be correct. If the strings are put on correctly on your guitar, the pitch will go up when the tuning peg is turned away from you, and lower when the peg is turned towards you. When the first string is in tune listen to the sound of the second string on the tuner, and repeat the same procedure until all six strings are in tune.
An online guitar tuner is right here

With the strings loose tune the open fifth string up to the pitch of A.

Next tune the fourth string up until it has the same sound as the fifth string held down at the fifth fret.

Next tune the third string up until it has the same sound as the fourth string held down at the fifth fret.

Next tune the second string up until it has the same sound as the third string held down at the fourth fret.

Next tune the first string up until it has the same sound as the second string held down at the fifth fret.

Lastly tune the sixth string up until, when it's plucked simultaneously with the first string, it has the same sound but two octaves lower.

Note: Tuning a guitar is not as hard as you might think, but it will take some practice. So if it takes a while to tune the guitar the first couple of times, do not get frustrated; it will become easier in the future.

You can tune a guitar to a keyboard instrument if you have one handy or you can use one of the variety of devices that goes under the name of "guitar tuner". First an explanation of the manual method of tuning your guitar. Using a tuning fork which vibrates at 440 Hz, you tune A on the fifth fret of the first string. More info at How To Use A Guitar Tuner

Some people find it much easier to use a computer guitar tuner to tune their guitar visually. Here is a link to the AP Guitar Tuner which you can install on your computer. It's easy to use, and if you're not confident about your ear for tuning, the visual tools make it a breeze!

And last, but not least is the free online guitar tuner from Get Tuned which features lots of alternate guitar tunings such as the ever-popular drop D tuning.

Of course, if you are not always near a computer when you need to tune your guitar, the Qwik Tune digital quartz tuner is by far the best of the carry-in-ya-pocket tuners. It features a 1/4" jack for electric tuning and an "Electronic Pitchpipe" internal speaker that plays E, A, D, G and B tones for tuning reference. A visual meter informs the user which string they are attempting to tune, even if the guitar is severely out of tune. High-sensitivity Microphone for extremely precise, accurate tuning. Fast Meter & 3 LEDs.

OPEN G TUNING - D G D G B D
The Open G tuning is over one hundred years old. In the nineteenth century a popular guitar piece called Spanish Fandango was played using Open G tuning, so it has been called the Spanish Tuning ever since. Hawaiian Slack Key players called it "Taro Patch". Blues guitarists liked Open G because it gave them the ability to lay a slide across all the strings and sound a major chord.

Here are some tabs of chords in Open G tuning. The x marks strings that are not played. Wher the two lowest strings are barred, use your thumb wrapped over the top of the neck.

MAJOR CHORDS
A
G|--2--------------------
B|-----2-----------------
G|--------2--------------
D|-----------2-----------
G|--------------2--------
D|-----------------2-----

Bb
G|--3--------------------
B|-----3-----------------
G|--------3--------------
D|-----------3-----------
G|--------------3--------
D|-----------------3-----

B
G|--4--------------------
B|-----4-----------------
G|--------4--------------
D|-----------4-----------
G|--------------4--------
D|-----------------4-----

C
G|--5--------------------
B|-----5-----------------
G|--------5--------------
D|-----------5-----------
G|--------------5--------
D|-----------------5-----

D
G|--7--------------------
B|-----7-----------------
G|--------7--------------
D|-----------7-----------
G|--------------7--------
D|-----------------7-----

E
G|--9--------------------
B|-----9-----------------
G|--------9--------------
D|-----------9-----------
G|--------------9--------
D|-----------------9-----

F
G|--10--------------------
B|-----10-----------------
G|--------10--------------
D|-----------10-----------
G|--------------10--------
D|-----------------10-----

G
G|--0--------------------
B|-----0-----------------
G|--------0--------------
D|-----------0-----------
G|--------------0--------
D|-----------------0-----

MINOR CHORDS
Am
G|--x--------------------
B|-----1-----------------
G|--------2--------------
D|-----------2-----------
G|--------------2--------
D|-----------------2-----

Bbm
G|--x--------------------
B|-----2-----------------
G|--------3--------------
D|-----------3-----------
G|--------------3--------
D|-----------------3-----

Bm
G|--x--------------------
B|-----3-----------------
G|--------4--------------
D|-----------4-----------
G|--------------4--------
D|-----------------4-----

Cm
G|--x--------------------
B|-----4-----------------
G|--------5--------------
D|-----------5-----------
G|--------------5--------
D|-----------------5-----

Dm
G|--x--------------------
B|-----6-----------------
G|--------7--------------
D|-----------7-----------
G|--------------7--------
D|-----------------7-----

Em
G|--x--------------------
B|-----8-----------------
G|--------9--------------
D|-----------9-----------
G|--------------x--------
D|-----------------9-----

Fm
G|--x--------------------
B|-----9-----------------
G|--------10--------------
D|-----------10-----------
G|--------------x--------
D|-----------------10-----

Gm
G|--0--------------------
B|-----3-----------------
G|--------3--------------
D|-----------0-----------
G|--------------0--------
D|-----------------0-----

SEVENTHS
A7
G|--2--------------------
B|-----2-----------------
G|--------2--------------
D|-----------5-----------
G|--------------2--------
D|-----------------2-----

Bb7
G|--3--------------------
B|-----3-----------------
G|--------6--------------
D|-----------6-----------
G|--------------3--------
D|-----------------3-----

C7
G|--5--------------------
B|-----5-----------------
G|--------5--------------
D|-----------8-----------
G|--------------5--------
D|-----------------5-----

D7
G|--7--------------------
B|-----7-----------------
G|--------7--------------
D|-----------10-----------
G|--------------7--------
D|-----------------7-----

E7
G|--9--------------------
B|-----9-----------------
G|--------9--------------
D|-----------12-----------
G|--------------9--------
D|-----------------9-----

F7
G|--10--------------------
B|-----10-----------------
G|--------10--------------
D|-----------13-----------
G|--------------10--------
D|-----------------10-----

G7
G|--0--------------------
B|-----0-----------------
G|--------0--------------
D|-----------3-----------
G|--------------0--------
D|-----------------0-----

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The Best Acoustic Guitar Tuner
Tuning your acoustic guitar is the first step in your guitar playing career. Whether by skill, talent or technology, you must be able to get your guitar into tune.

The modern twelve string guitar appeared in The United States at the end of the nineteenth century. Nobody knows who first got the idea of making a guitar with double courses of strings. Some guitar historians see the twelve string as an extension of the long tradition of coursed instruments in Italian musical history. But many people think that Mexico, with its tradition of variations on the conventional guitar is the more likely source.
A six string guitar has 6 strings tuned to E A D G B E. The twelve string guitar has six pairs of strings, each pair tuned to the same note but E A D G are tuned an octave apart. B and E are in unison.
Let's look at the strings:
E (First String)
e

B
b

G
g

D
d Tuned to the 2nd string - 3rd fret

A
a Tuned to the 3rd string - 2nd fret

E
e (Sixth String)Tuned to the 4th string - 2nd fret

So use your guitar tuner or pitch pipe to get six of the strings in tune as normal. Then go back and do your additional 1st, 2nd and 3rd strings as normal. Then tune your additional 6th, 5th and 4th strings an octave higher.
I'm afraid the final step is to go back again and repeat the tuning process a number of times until your guitar is in tune because the neck of a twelve string guitar comes under alot of stress, and the pithch of the strings will change!
It's harder to explain than it is to do, so after you've done it once, it'll be a snap!
Once you have got the hang of it, there's a neat trick for playing twelve string guitar: removing the higher octave from the E A and D strings makes playing running bass lines easier, without losing that distinctive twelve string sound in strumming passages.
The twelve string guitar is great for strumming along accompanying songs but it does not lend itself to the techniques involved in playing lead guitar.
The twelve string guitar has a warm, ear-friendly sound which draws attention to the guitar even behind a group of singers.
Twelve string guitars are usually acoustic, although some lead guitarists have used electric twelve string guitars from time to time.
Guitarists who favored the twelve string include early blues guitarists Blind Willie McTell and Lead Belly, folk singer Bob Gibson and sixties folk-rock innovators, The Byrds. It has not had any great popularity since the seventies when Led Zeppelin and The Eagles recorded 'Stairway to Heaven' and 'Hotel California'.