Machine embroidery basics

If you are just getting started there are a few things you need to know:
Start off by embroidering onto stable fabrics, stretchy knit fabrics are hard to hoop and you will need a bit of practice before you can move onto them. Your fabric should be firm in your hoop but not stretched or distorted. Hoop the stabilizer with the fabric. I did not know this when I first started out and I would just put a bit of stabilizer under where my embroidery design was going on the fabric. The result when I think back now wasn't surprising, it did not stabilize the fabric and my results were not good.

Make sure your design is suitable for the fabric you are stitching on. Don't use really dense heavy designs on light weight fabric, and don't try to stitch a delicate design onto a towel, it will completely disappear int the pile.
Chose the correct stabilizer. Cuttaway is recommended for unstable fabrics like knits it is also better if the garment is being washed often and is softer to touch than tearaway. Tearaway is good if you don't want the stabilizer showing on the back because it can be almost completely removed, it is more brittle to touch and therefore can be scratchy against the skin. Washaway is perfect for free standing lace or 3D designs. A water soluble topper is essential on top of towels and fabric with a pile.
Remember to be patient, and have fun.

Whether you digitize your own designs or you purchase them from the internet or on CD it is wise to have some understanding of the physics of machine embroidery
The first and most important thing to remember is there is no such thing as perfect embroidery, there is only quality embroidery. Embroidery is abstract in nature, this is because all stitches are straight. A line or design may appear curved but in fact it is made up of straight stitches. machine embroidery also limited by the number of stitches that will fit into the fabric, remember we are embroidering into fabric not onto it. It is not like ink on paper therefore there is a limit to how much detail can be added to an embroidery design.
The type of fabric you wish to stitch a design into needs to be given serious consideration when you are choosing an appropriate design. A light weight fabric or a light knit requires a low density design, one color work (redwork) will work well on these fabrics, a dense or large design will not produce good results on stretch fabric, remember that the fabric stretches but the embroidery design does not, this can cause distortion in the fabric. When stitching onto towel or high pile fabric a dense embroidery design will work better, a light design will simply disappear into the pile and will become nearly invisible. It is very important to use a soluble topper when stitching into toweling.
Lastly proper hooping is very important. Choosing the correct stabilizer for the fabric and design is very important. When hooping up you may hear that that your fabric need to be as "tight as a drum" This is not true, your fabric should be firmly hooped but the fabric should not be distorted. Firm hooping will prevent registry errors in the stitch out, this means the design is digitized well but the stitching process is not correct, this can result in outlines being off, or the design being misaligned. Try to perfect your hooping technique so that you don't need to pull your fabric around once it is in the hoop. If you do need to adjust it make sure that you have not distorted the fabric.