My brother always gets all annoyed with his Rubik's cube because he can "do" one side. That's fine, it's exactly how you start. But here's the thing. This is NOT necessarily correct:
THIS is how you know it's correct:
This photo is borrowed from Ajae - see how the green is all done but there's orange, red, and yellow all next to each other? That can't be correct.
So that is my (very well-known) secret for you. Start with one side - but make sure ALL the colors line up. The rest is CAKE!