Why don't you start by reshingling your garage or something? Simple gable to gable roof that's not too big. No valleys, dormers, or penetrations / flashing details to deal with - and they usually aren't too high or steep.
You'll need a few things - ladder, hand tools (tape measure, chalk line, hammer, pry bar, maybe a cats paw, razor knife with
hook blades), tear off shovel, scoop shovel, wheel barrow, circular saw, big poly tarp, roofing materials, cushions, and your a$$
Find an old couch, and pull the cushions out. Peel the white fuzzy stuff off - get down to yellow foam. That is to sit on, and sit materials on. It sticks to shingles like glue.
Wear tennis shoes you don't care about. I used skateboard shoes before I got cougar paws.
Don't bother with a nail gun for this unless you want to spend about $500 on the rig - then buy the Hitachi NV45AB2 - the only roofing gun worth buying IMHO - and one of those lightweight 100 foot hoses. Regular air hoses are worthless on the roof. These are the lightweight 100 foot ones.
you'll need a way to get rid of the tear off. For a garage, a friend with a trailer will probably do.
Spread the tarp out below where you're tearing off. Bust off the ridge cap, and work top down. Tear off down to decking, and pull nails. Try not to pull out decking nails. Replace any bad decking, and refasten any decking nails that got pulled.
Tear off onto the tarp, then scoop shovel into wheel barrow, and roll it onto the trailer and dump. Repeat until you're done - take a break, then finish the pile
Hopefully, you've been doing some research too. Buy a book, use the internet, talk to friends. Figure out how to avoid the short course at the top by adjusting exposure for one. Some guys use the shingles as a guide to keep lines straight on smaller roofs. Don't try it. I pop lines every other course (every 10" without adjustments for top course). Keep in mind how much you're overlapping the edges when popping lines.
When installing, work from the bottom up. 3' roll of ice / water shield at the bottom. lay it where it goes, then peel / stick. Felt the roof, pop lines, run starter strip, then shingle and put on the ridge cap.
There are instructions on the packages that tell you how to install them, but make sure you follow local codes too. Around here, you don't need a permit to re-roof, and there are no additional fastening requirements.
Whatever you do - don't high nail! Noobs often think that going higher keeps the penetrations further from water, but it also makes you miss the course below it.
When you're walking on a felted roof, walk on the caps (nails)! Felt is SLICK! and 15# tears pretty easily - either way, WHOOSH! you're on the ground.
Now you'll have some idea of what you're in for. Now imagine doing that on a 12:12 (45 degree angle) second story roof.
You'll be able to admire your handy work for a long time if you do it right - at least until you roof for a while and can pick apart everything you did wrong

More likely, you'll just admire it for a long time after you go back to the office job though ... it's really hard work!
FYI - I can tear off and install the average 2 car garage in about a day and a half - 2 days by myself from pulling up to the job, to 100% done and cleaned up. It's slow by some guy's standards. I know guys that can do it in a day every time but I don't try to kill myself toting shingles or tear off.