I have an iMac that I have had for about 2 years. It has alot of stuff on it, and everything that I need I have put on an external hard drive. I was wanted to completely erase everything off of the imac now. I have the install discs it came with. But I never set up my time machine. I have googled to find how to do this and all I see is through time machine but I never set it up. So how do I completely restore it so it is fresh with nothing on it besides the OS?