I think all the physicsl prep has been well covered, but almost as important is the mental prep. Your first century is a leap into the unknown, will you bonk, what if I get too many aches and pains to continue etc etc. So here are a few tips that I found useful in doing my first century earlier this year.

1) Know your route, maybe put two of your 50 mile routes together.
2) Don't try something with too many hills.
3) Bring plenty of water and things to chomp on
4) Choose a nice day to do it
5) If possible ride with someone who's done a Century before
6) Have patience, it'll be a long day, and don't give up