The most recent edition of The Family Handyman has some good, general info on doing a roof in the LOWE'S Section. The bookstore and library are good sources for info too (and they're both open on Saturday's). For what it's worth, obviously, you'll save some $$$ even if you only end up doing the demolition (if this is necessary). But at the same time, to do the job right the first time, SPEND some $$$! Get a good book that's going to help you. Or, find a roofing laborer that does side work to get you started. In any event, for a small fee, they'll act as a consultant and/or get you started. All the way around, it's the best way to go in my opinion. (If you have anyone on the roof, make sure your Homeowner's Insurance is up to date.)
In the Home Improvement business, for any non-professional like you and me, a little money spent is well worth it! Just think of the cost if the job has to be re-done by a pro. My $.02. And my best to ya.