Youtube videos are great, but I found myself wanting text as well. If you're willing to spend just a little $, try Zinn and the Art of Mountain Bike Maintenance. I found it helps me to be able to read through the whole process and then do the work rather than just watching someone do it on a video.
Since ordering this book I've fixed my brakes and 2 drivetrains (front and rear derailleurs both). It's been super helpful! But at the same time, I'm also watching the youtube videos to see what it looks like in action.
Make friends with a mechanic at the local shop(buying them tasty beer helps) or pick up a copy of this http://www.amazon.com/Zinn-Art-Mountain-Bike-Maintenance/dp/1934030597
Also if there is a riding or trail building/advocacy club in your area they would possibly be just as useful as the local shop mechanic and will take you for a tour of the trails after getting your bike dialed in(again bring them beer).
FWIW, I'm a former shop mechanic/manager and currently a board member of the local trail building/maintenance/advocacy non-profit. I could go into great detail on your questions but noone likes a wall of text.
Of the three the giant. Walmart makes bikes for kids who wish their parents let them go mountain biking. You should check out bikesdirect.com. You could get a new entry level bike for 320 that would be adequate.
http://www.amazon.com/Zinn-Art-Mountain-Bike-Maintenance/dp/1934030597.
Mountain biking is a lot rougher than road cycling so get that. Will save you a lot in fees from the bike shop.
As far as wrenching in general goes, we've got /r/bikewrench you can refer to. Also, I've heard really good things about Zinn and the Art of Mountain Bike Maintenance. Don't have a copy yet myself but it's been on my wish list.
Buy this book: http://www.amazon.com/Zinn-Art-Mountain-Bike-Maintenance/dp/1934030597
I have a version that is at least 10 years old that I still reference for various maintenance tasks.
Get an idea of WHAT people are suggesting in this thread, then use this book to figure out HOW to do it.
It's funny to see this. Just this morning I was looking through old photos and came across my first real MTB which I bought in December 2017: https://nuxx.net/gallery/d/71823-2/IMG_0364.jpg
Bike-wise, as others have said, get what you can afford. Mountain biking is really all about riding a bike in the woods, and there's almost nothing that won't be to some degree fun, outside of a road bike. So get what you can afford, ride it, don't fall prey to upgrade-itis, but do know that if you REALLY like it you'll probably want something better later on. Only by riding will you find the shortcomings and figure out what those are.
Gear-wise, I'd start with the following:
Zinn & the Art of Mountain Bike Maintenance. Do you watch GMBN?
I highly recommend Zinn and the Art of Mountain Bike Maintenance
I've never read it, but I've heard good things about this book. I see it get recommended a lot.
Also, there is this site. A little weird to navigate but it has lots of great technical information.
Wow! Lots of information!
Here are some pictures of my bike: http://imgur.com/share/a/Y3JLT Sorry if they are bad.
I can get a wrench out and tighten things, but beyond that I have very little knowledge of how a bike works. Do you think I should read this book?
As for camping, there would probably be an adult riding with us. I also have a learner's permit, which is a form of state ID, right?
like /u/AnelloGrande said It sounds like an issue with the tension in your rear derailleur. Rather than a bike shop though, I'd recommend grabbing something like Zinn and the Art of Mountain Bike Maintenance for $20 and you'll learn how easy it is to do all these repairs yourself. For derailleur tension it's as simple as using the cable barrel adjuster (this thing).
I actually just bought this: http://www.amazon.com/Zinn-Art-Mountain-Bike-Maintenance/dp/1934030597/ref=pd_sim_b_1?ie=UTF8&refRID=0AXGS91NBG6ZV6HEYTPD
Hopefully it'll help
this helps me a lot
http://www.amazon.com/Zinn-Art-Mountain-Bike-Maintenance/dp/1934030597
Oil the chain, go through all the gears, buy this book.
This is a great book. It covers pretty much everything you need to know and the explanations are really good. I keep it in my garage and it comes in handy. http://www.amazon.com/Zinn-Art-Mountain-Bike-Maintenance/dp/1934030597/ref=sr_1_3?ie=UTF8&qid=1338911183&sr=8-3
Also PinkBike tech tuesdays are pretty good as well. Lots of videos and info. Here is a link to the latest one but if you scroll down there are links to all the other articles. http://www.pinkbike.com/news/Tech-Tuesday-Service-and-Re-Grease-a-Cartridge-Bearing-Headset.html