Look up a pattern that you are interested in and see what yarn other people have used, how it looks in different colors, etc. If you have yarn that you don't know what it wants to be look up the yarn and see what others have made with it.
I used it mostly as a resource. It's the first place I go to look up patterns or find something to with a yarn I have, etc. I bought some Encore DK the other day to make fingerless mittens, but didn't have a pattern specifically. I typed in DK and then found patterns that had been made with that weight yarn and I found several that worked! All in one place!