Welcome to the Apple Support Communities
You can install Windows 7 on your computer without any problem. First, determine if you need Boot Camp or a virtual machine > https://discussions.apple.com/docs/DOC-3321
If you need Boot Camp, get a Windows 7 retail DVD, open Boot Camp Assistant and follow the steps to install Windows. If you want all the steps to install it, you can print this PDF file > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.7.pdf. Windows 8 support will come with a OS X update.
If you need a virtual machine, you can use Parallels, VMware Fusion or VirtualBox. All of them are paid except VirtualBox, and they work well with Windows