Get degreaser specially made for car engine bays and read the instructions. Try not to cheap out and get something environmentally friendly. Make sure to try to cover the alternator and battery as best as you can with plastic bags or something. If it does get wet, dont worry about it, just dry it off with a towel and maybe a hairdryer or something and dont turn on the car for a while.

good luck and have fun, i want to do this again someday soon too.