I would suggest learning to adjust the setup on your bike before anything else.
If you're mechanically minded, get your hands dirty and have a play around. If you're not, a bicycle is a great place to start your learning, and you can always take it to a bike store if it frustrates you too much. There are a lot of youtube videos on the subject if you prefer them over books:Adjusting brakes:
Adjusting your rear derailleur Adjusting your rear derailleur, which is often the cause of gear troubles.:
You'll first want to check the back wheel is true and aligned correctly. Sometimes going over gutters and bumps can knock the wheel loose which knocks everything else out of alignment.
Steel cables will stretch over time, especially if you store them under tension like leaving your bike in a low gear overnight. It's worthwhile learning how to adjust them as they need it done periodically.
You'll need some basic tools, depending on your bike, generally:
- a shifter (adjustable/shifting spanner) or a socket set
- phillips head screwdriver
- allen keys
After you do any work, do a short test ride out of traffic and harms way in case something goes wrong e.g. your brakes don't work as expected or your wheel locks up.
Good luck and happy cycling!