Gentoo is amzingly hard to install ... only and only recommended for experienced users, for those who are new to linux like you and me, go for fedora core 4 ... very friendly if you ask me ...
Anyhow, you need to partition your hard drive if you want to install and run windows or linux on the same computer ... you might be gifted with money and be able to buy a program like Partition Magic or you can use the free program called Qtparted which is included on nearly every single linux live cd ...