Is it in-dash, 6 disc changer, or in the trunk?
If it's in the dash I'm guessing it takes up two DIN slots. Most aftermarket head units are single-DIN, but you can get double DIN as well. If you get single DIN, then just block up the other DIN slot with some kind of plastic.
Get an Alpine head unit, they are the best. Or for cheaper get Pioneer or Kenwood.
Anyway, you'll need to take the thing out of the dash, and put the new one in. For this you need a wiring harness (sold specific to your model).
Anyway if you have no experience I suggest getting the head unit installed professionally.
Just go to Best Buy and they will take out your old radio and put in a new one. Kinda expensive though.
Also as a side note I recommend getting a unit that plays MP3/WMA. Definitely worth the money.