Mechanical disk brakes perform best when they are regularly maintained. When you first get your bike, the cables will stretch over time, and the brakes pads will slowly wear out. As this happens, it's important to adjust your brakes to compensate for this, to ensure that you have safe and reliable stopping power.

How to do a brake tune up:
Disk brakes can make two different noises. Sometimes they make a loud squealing noise during stopping. It can happen if there's some rotor contamination. Sometimes they make a much quieter noise that makes more of a ringing sound. If that's the case, you'll need to make an adjustment to the brakes, so that the pads aren't rubbing on the brake rotor.

Park Tool has a very comprehensive video on how to do adjustments to mechanical disk brakes. The brakes on our bikes are a single lever with single adjustment:

This is the timestamp for your type of brakes:

How to fix disk brake rotor warp:
Although it is annoying, it is normal for disk brake rotors to occasionally get warped during riding or handling of the bike. Park Tool makes a great video that covers all aspects of truing disk brake rotors:
How to True a Bicycle Disc Brake Rotor (Park Tool 10:55)

You can use a dedicated tool like the Park Tool DT-2 rotor truing tool, but I have also used the jaws of smaller Crescent wrench to do corrections.

When you are done, there should be a thin gap of a millimeter or less between the rotor and the brake pad material:

How to fix brake rotor contamination:
Get a clean shop towel, wet it with some isopropyl alcohol and wipe down both sides of the the brake rotor. Sometimes the edges of the rotor can be sharp. Make sure to never touch the rotor surface with your bare hands. Where you ride can make a difference too. City streets where cars drip oil, or dusty gravel roads can both contribute to rotor noise.