There are a few ways to approach shooting a wedding.

The traditional/formal approach is a very structured, pretty much scripted method to get salable photos. You pose everyone, capture the main moments, don't worry about random moments as much. This approach is good if you're offering your photography a la carte (no set packages.) You don't shoot things that don't directly sell.

The photojournalism approach is to blend into the background, no posed pictures, mostly using existing light (little or no flash), lots of details and always watching the guests and couple for expressions. This is great if you "sell" on this style and offer only packages which are bought before the wedding.

Neither approach is wrong, they are just different.

The traditional approach requires a good knowledge of posing and the ability to get in and get people's attention - get them to the right places and looking in the right direction. Its not all stiff poses - there can be a lot of creativity to this approach and have casual, relaxed looks, but it takes direction and knowledge. While many couples will say they want detail shots and non-posed casual shots, if they are buying individual prints afterwards, most professionals find couples really only buy a select number of posed shots showing specific people and all the other shots go unprinted.

The photojournalism approach takes an excellent knowledge of how a wedding proceeds so that you know precisely where and when to take pictures. It would also be ideal to have two photographers running around making sure everything is covered, lots of reaction shots of the guests, documenting all the details and so on. It also takes an excellent eye for detail, composition and a great sense of timing.

A third way to make use of both styles, posing people where needed but also getting as many detail shots and reaction shots as you can. This is great when you have a two photographers - one photo-journalistic and one more formal. Again, with the combination of styles, you probably want to have sold specific packages with custom albums you can design to use as many of your shots as practical and not count on the couple buying prints of all the shots.

As a rule, people look better turned 20 to 45 degrees to the side, with their weight on the foot farther from the camera. If you have a group, divide them in the center, with each half turned towards the middle.

If people are sitting, again turn them just a bit to the side and try to get them to sit more on one bum cheek than the other.

As a rule, guys should lean their head to the lower shoulder, and girls to the higher shoulder.

My preference is to not have hands held in front of the crotch like the person has to pee badly, or crossed over the chest. I don't mind the arm hanging relaxed at the side of the body. Some photographers hate that and would rather use the "fig leaf."

That said, its a good idea to "bend anything that bends" just a bit so its not all "straight sticks."

A relaxed look for hands and arms is to put a hand in a pocket (with thumb out so it doesn't look like the person is playing "pocket pool", or with a thumb hooked into the pocket or on a belt.

There is a great resource for leaning posing below in the links.

Digital or film

Digital seem to be taking over the industry, although many people still use film.

Film actually is easier to deal with, shoot, print, sort, present. But it also costs more in the long run, isn't as flexible, and if you shooting 35mm - the quality just isn't as good any more.

Digital has some hidden costs - like all the hard drives you'll need for storing pics, and costs a bit more to get into. You'll upgrade more often as well. But if you have to copy eyes from one pic to another, or put an extra person in, or take someone out (I've done all three) digital is way more flexible.

Either way, you'll need at least one SLR camera body - and you really should have two. Point and shoots are not wedding photography cameras.

Lenses

You'll need a wide to short-tele zoom lens something like 18mm to 60mm on most digital cameras, or 35 to 70mm on full-frame or film cameras. You should also have a slightly longer telephoto or zoom lens. Ideally, all your lenses will be a constant f2.8 or faster. A handy, and cheap, option is to add a basic 50mm f1.8. On most digital bodies you have a fast "75mm" equivalent lens.

Flash

A flash is something you'll also need, get a professional level one, the small built in flashes and lower level on-camera flashes just don't cut it at weddings. And get a second one as back up.

Computer

Even if you're not shooting digital, a computer should be on hand for your invoicing, contracts, emails, etc. But if you're shooting digital, you'll need a fast computer with as much hard drive and ram (especially ram) as you can afford.

A good sorting program like Adobe Lightroom (a current favorite of mine) is really handy for productivity when sorting, eliminating and presenting images.

Backup...

If you're shooting digital, once you've done the shoot, make sure you ALWAYS have the images in at least two places. Internal and external hard drives, CD's, DVD's etc. If its not in two places - it may well not exist at all. You really don't want to potentially loose thousands of dollars for want of a $100 back up drive.

These are the basics you'll need. After doing it for a shot time you'll quickly discover a lot more gear you also "need" to have.

First and foremost, even if you're just doing this for fiends (or friends of friends) or doing it VERY part time, make sure you have a good contract in place and specify exactly what you are providing, what the couple can expect, how much they will be charged and when payments must be made. Check out the link provided below for sample contracts.

It is also a really good idea to have a second bank account to keep your payments and expenses separate from your personal life.

Keep track of ALL your expenses such as mileage on your car (including when you go see the couple to "sell" the wedding), how much of your phone time is for business related needs, your gear and supplies. The more you track this stuff, the more you can write off at tax time.

Learn about selling. You need to "sell" the photography, both to get the shoot, and possibly to sell the most prints possible afterwards. The more you understand about selling, the better you'll probably do.