And adding on to Delta's good advice, after you've measured yourself (or better, have someone measure you-- it should be more accurate) choose your pattern size. Then lay out the pattern and measure the pattern pieces at bust, waist and hips. Subtract 5/8" (1.5 cm) for each seam allowance. Double the resulting number and it should be bigger than your measurements. The extra amount is "ease", and it's a hard thing to judge when you're new to sewing (but it gets easier, I promise!)
There are a couple of ways to think about how much ease you want with a pattern. If you've got a favorite dress or blouse, you can measure that at bust, waist and hips and see if you think there's way too much ease, or not enough ease, in the pattern. Or you can just loop the tape measure around you with the pattern's measurements and see if you really want 12" of ease in the bust (or whatever the difference between you and the pattern's measurement might be).
More help here, perhaps:
http://www.simplicity.com/index.cfm?page=fitHelp_main.html
http://www.taunton.com/threads/pages/t00188.asp
http://www.taunton.com/threads/pages/t00136.asp
http://www.taunton.com/threads/pages/t00004.asp
As for cheap... good quality fabric (which is not necessarily the same as expensive fabric) sews and looks and hangs together so much better than cheap stuff.
And yes, you can probably buy ready to wear more cheaply than you can make most garments. But you can't get made to measure custom garments as inexpensively as you can sew them. Work for the custom made look, not the bargain ready to wear look, and you'll save $$$$$