Two ways. Easiest way is to go on line to gmail and do it on the computer.
You can do it by phone also, it's not hard. Go to your contacts, long press on the contact, and a menu should pop up with edit on it. Select that and make your changes.
At least off the top of my head I think that's how it's done, someone will correct me if I'm wrong. You may have to select the contact and then menu button and then edit. One of those ways should work.
Bill is right if your using motoblur/stock rom. Liberty can still sync to facebook but the process is a bit different. Since your new I assume your still on stock