What kind of top are you planning to make? What weight is the cotton? If it's thin you may only have enough for a short sleeve or tank top. If you got the yarn at a yarn store, they'll be able to help you pick a simple pattern. You can also check your local library for knitting books and magazines. There's a Vogue series with easy knits. Here's a site with free patterns:

http://www.knittingpatterncentral.com/directory.php

How about a tank top? That was my first garment (after I made two ponchos). Your local yarn shop personnel should be able to help you pick out a pattern. I made mine with Patons Fresco (see link).

Here is a link that might be useful: Top