I have to echo artizzle that you'll be much better off with an entry level complete bike than you will be with a conversion, especially when you don't know much about bikes or fixed gear components to start with. The Mercier is hard to beat. You can also look for a used Pista/Fuji track/KHS. One of the best things about buying new is that you can get precisely the size you want, and won't be tempted to put up with a slightly ill-fitting bike because it's a good deal. A good fitting bike is more important than any bling components.

Also, put a front brake on it, no matter what all your cool fixie friends say.