You will find that most patterns come multi-sized, like 8-10-12, allowing several sizes to be made from the same pattern. This is also helpful in making alterations. Choose the pattern that includes ranges that include your measurements.
You should choose patterns that are suitable to your sewing experience. As you are starting out, look for easy to sew patterns. These will be less likely to include more difficult features like pleats and collars. The back of the pattern envelope tells you how many pattern pieces it includes. Remember, the fewer the pieces, the easier the pattern.
Look through the pattern books at the fabric store to find the pattern you like. Let your creativity shine when doing so. Envision the garments in colors, prints, or designs other than what is pictured. Look at style and construction rather than fabrics used in the pattern photos.
Once you find the pattern you like, look for it in the big pattern drawers where patterns are filed by pattern number. The different brands will be in different drawers. If you want Simplicity pattern 3699, go to the Simplicity drawers, then look for the drawer that includes 3699. Once you find the tab where your pattern is, be sure you pull out the correct size. All sizes for a particular pattern number will be filed in the same slot.
TIP: Never pay full price for a pattern. They're almost always on sale somehwere, sometimes for as little as $1. Hobby Lobby and Joann's sell certain pattern brands at 40% off every day.