You have to set up a dual boot.. meaning 2 OSes on your system instead of one.. and you can choose to boot (start up) in one, or the other...
The Apple utility that leads you through setting up a dual boot is called Bootcamp.
Just do some google searches and read about setting up Bootcamp. You'll need a licensed version of Windows 7, and then just use Bootcamp to get it installed.
When booted into Windows, it will be like any other PC... meaning you have to take care of the Windos install like any other PC with all the same +'s and -'s.