Wash it in hot, soapy water first. Allow it to air dry, and don't touch the inside. The body should have an outside film, this should only be peeled off once the shell is completed.
If you want any stripes/designs, or if it doesn't come with window masks, mask them off with Tamiya tape at this point. I would advise though, since it's your first shell, go for a single colour scheme.
I use Tamiya Polycarbonate sprays, I find them the easiest to work with. You'll want a big cardboard box, and put the body in upside down, ready to paint. Make sure you do it outside, or have adequate ventilation.
When you're ready, spray a very, very fine coat each time. This should be no more than a light mist. They each take about an hour to dry, so you should get it done in 2/3 days. When you're finished and happy, you can peel the outer film off.
Finally, if the body needs cutting, use lexan scissors. A number of RC manufacturers sell these. Then apply your decals/stickers (I think Tamiya still do slide on), and use a body reamer (only about $15, they'll serve you well!) to drill the holes for the body posts. Make sure you line it up correctly first.
Then, you should be done.