I like the Park Tool Repair Help pages for learning repairs. Sometimes, a youtube video helps on complicated repairs.

The rear derailleur adjustment page starts from the beginning, setting the high and low limit screws, then adjusting the shifting. Do the whole thing so you know it's correct. Then you'll understand how to adjust it quickly in the future. It's easy after you do it the first time.

If the low limit screw is set too far, it'll allow the chain to go past the large cog, into the spokes. Then the derailleur catches on the spokes, and rips it off the bike. It'll need spokes, derailleur, and often the frame is damaged too. An expensive problem.