The most important thing to look for when choosing fabric for a dress is the right texture, weight and drape. I think many beginners either choose fabric that is too stiff (which makes clothes look boxy), or too slippery (which is hard to work with). When you see a fabric that catches your eye, roll it off the bolt and hold it up to see how it hangs. You can even go so far as to wrap it around your body to see if it has the right feel. Look for a fabric that is light to medium weight, which is probably best for most dresses (but of course it depends on your particular pattern). One word of caution, I advise against choosing quilting cottons for dresses. Even though they are super cute, they're generally too stiff to make good-looking clothes. Another word of caution is to avoid novelty fabrics or anything too sheer, slinky or slippery because they can be very difficult for beginners to sew.

Fit is the most common pitfall for home sewers. To remedy this, I suggest taking the time to make a muslin, which is a test run of your pattern. That means you'll sew the dress just like the real thing, but make it in an inexpensive fabric that you don't mind "wasting." That will allow you to test out all the elements of your pattern so that you'll work out any kinks and have everything down to a science by the time you sew the real dress in your actual fabric. It sounds like extra work but it's a great way to learn and can really help your finished projects be more successful. 

How do you keep layers of fabric smooth and even when sewing?  

Pin, pin, pin! Pin your layers of fabric together with straight pins, inserting them perpendicular to the raw edge of the fabric. Place a pin every few inches or so, and your fabric shouldn't shift or bunch at all. Also, be sure you keep the fabric smoothed out as you work rather than bunched up on your lap or to the side. It's easy to accidentally sew over more than one layer, but taking an extra second to keep the fabric smoothed out will avoid this pitfall. 

Stand up straight in front of a mirror, wearing nothing but the undergarments you plan to wear under the clothes you're sewing. Then just wrap a tape measure around the portion of the body you want to measure, pulling it slightly taut but not so tight that the tape stretches or pulls in the skin at all. The safest bet is to always measure the fullest part of whatever you're measuring. So, for the bust and hips, be sure to measure the fullest spot so that you don't risk your garments being too small. For the waist measurement, measure the spot on your waist where the waist of the garment will actually be. The best way to do this is to have someone else help you because it can be hard to get accurate measurements on your own body, since you have to bend and move in order to do it (which means you won't be standing up straight). 

My favorite method is to baste a seam where you want to install the zipper, then attach the zipper to the wrong side of the basted seam. Then you simply remove the basted stitches, and your zipper is done! It's the easiest thing ever. I made a video of this method showing how it's done, which you can find on my site Manhattan Craft Room.