It can be a bit of minefield out there and there really isn't any help for the a new user.
One site I used when I first started out was click here it's an excellent company that build high quality PC's. The key things to look out for are high spec graphics cards (google them if you're not sure) Good RAM (at least 2gb) and a high hard drive capacity. Beware however, because some of the computers don't come preloaded with operating systems, but they do quote a price with or without the OS's. I hope this is of some help.