Normally you buy a specific size pattern. Please note that pattern sizes have little or no relation to retail clothing sizes! Always buy patterns by your body measurements.
I strongly recommend first making up the pattern in cheap fabric (utlity muslin or thrift store sheets), make sure it fits, and alter your pattern accordingly. Very few of us are standard size in all respects. It's a lot easier to alter a pattern before you cut it out in your expensive fabric!
Most patterns come with instructions, but some are better than others.
There are a few patterns that are multi-size, showing which line to cut on for which size. For those, it's even more important to do a trial fitting in cheap fabric first.
If you haven't sewn before, consider taking a basic class, if you can find one.
Edited by OregonMouse (04/01/16 01:27 PM)
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey