Well, what model year is it and what OS is it running? Did it come with any restore CDs?
If it came with Lion or ML, it should have a recovery partition in which you can boot into. You then go to disk utility, erase the hard drive, and then reinstall the OS.
Hold command-R on boot and select the recovery partition. You then want to go into disk utility, find the macintosh HD, erase it. Then go back to the main window, and just reinstall OSX. Thats it.
OK, another question (sorry). I'm using a regular keyboard, one made for Windows, I know the "Windows" key is supposed to act as the Command key, but it isn't working. I'm lost.
FYI, I'm waiting for my Apple kb to come in, using this as an alternative for now.