N00B ;) What to do after a rebuild?

I'm 14 and have long been interested in doing IT repairs and support as I'm admittedly good with computers. I thought it would be best to reemble an old PC before I attempted building my own. I found an 7 or 8 year old PC in our attic. Firstly, how would you disemble a PC in stages (which components first etc.) and after putting it back together should I use it as a guinea pig for new programs or scavenge certain parts for a new system?