I have an iphone (model MA712LL, version 2.02). It's been jailbroken already, and it's full of files and stuff from the previous owner. I want to completely wipe it clean and start over. I want to use Sprint. I have no idea where to start. Can somebody please point me in the direction of the correct tutorials? Thanks so much.

Bad news, the phone you have is a GSM only iPhone (originally locked to AT&T, if I recall). It does not have a CDMA baseband and cannot be used on Sprint's network. As far as wiping clean, you can 'erase all content and settings' HOWEVER this will require you to restore iOS to the device before it will be usable again. That said, you will not be able to unlock the device to work on any network other than GSM networks, and even then, since this is the 2G iPhone, it will only work with 2G / EDGE networks.