Gentoo is a lot trickier to install than most distros. Unfortunately it isn't as simple as 'which Disc do I pop in?' Chances are you will need both the stages and packages CDs. As it has been a long time since I have tried Gentoo, I can't remember exactly what you will need to do. I do remember, though, the source that was invaluable for me as a Gentoo newbie:
The Gentoo Handbook
provides all the steps you will need to follow in order to get Gentoo up and running on your system. Read through the Handbook completely (you will probably want to print the most relevant parts off to have with you while you install). Make sure you allow yourself plenty of time (on the order of days) as most things need to compile.
Good luck in your excursion.