how do you make sure youre able to sell your computer to someone else in the best condition as possible?

how do i make sure that i erase everything ive installed and reset the comp to its original settings? is this what reformatting does? and does this get rid of possibly lurking viruses or crap thats accumlated over the months? i want it to work as fast as it did when i first got it



please excuse the noobness h: