Please do not take offense to this answer, but if you are not aware of this answer, I would surely contract out the job.
First, are we talking about composition shingles? Second, how many layers of shingles do you have on your roof currently? If you have two, then you need to remove one layer and removal of all layers is more easier than one layer.
When installing shingles on a decking with tar paper, you begin at the drip line or the bottom of the house laying a string of shingles.
Then you decide on the course or method of laying down shingles and begin on top of this first layer of shingles.
This is just too hard to explain over the Internet, I would go to a home center or roofing company and see if someone would assist you.
You know, the last thing I want to ever do on this site is to offend someone, and i feel that you may be thinking that I am doing that now. But, this is too much of a job and if you mess it up, and the roof leaks, who is to blame?