How do I reformat the hard drive?

I want to reformat the hard drive of my son’s computer and have never done it solo before. I just want to be sure I remove everything I need to prior to starting.

Is there an article or can you give me steps that I should follow to perform this properly? I don’t want to fdisk his machine without removing/saving everything I should prior to checking. He is running Win98SE, PIII 366mz, w/64mg ram.