The most common types of troubleshooting with all disc brakes are caliper alignment, truing the rotor, and excessive brake noise.
-Brake Caliper Alignment
A misaligned caliper can cause the brake "bite" to feel subpar and can give a weak feel to the lever pull. Also, a misaligned caliper may cause the rotor to rub the pads even when the brakes are not being applied which can be noisy and bothersome. To make this worse, if the rotor is not true, meaning it has a slight bend or warp, this can further exacerbate the issue. Fortunately, these are both very easy to correct!
The first step would be to get the bike in the air. A bike stand is ideal but hanging the bike by its saddle can also provide the needed elevation. You will want to be able to spin the wheel as you go to check alignment. Next, you will need a 5mm hex wrench to use when loosening the two caliper bolts for either a front or rear brake. (figure 1a/b)
Figure 1a (rear) Figure 1b (front)
You will want them just loose enough to allow the caliper to have side-to-side movement on either bolt. A good way to start the alignment process is to loosen the caliper bolts, then grab that same brake's lever and while squeezing, snug the two bolts. This will give an initial alignment. To fine-tune it, you will then look through the "window" (figure 2) of the caliper, and your goal is to adjust it so that there is visible daylight showing between the pad and rotor on both sides. (figure 3) You can adjust this by slightly loosening one bolt at a time and applying pressure to the caliper with your fingers in the direction you want it to move. Then, while still applying pressure, snug the bolt back down. This can be repeated on either bolt until the spacing looks like Figure 3. Be careful of your fingers and spin the wheel to ensure the rotor does not contact the pads. If you are satisfied, fully snug both brake caliper bolts.
Note: For mechanical brakes, follow the specific tips for adjustment in the "Mechanical" brake video as the alignment process is the same but some mechanical brakes may have extra adjustments to dial in how the pads contact the rotor and give the desired performance.
Below is a video showing this process:
Hydraulic: https://www.youtube.com/watch?v=uk_nC9anQcM&t=168s
Mechanical: https://www.youtube.com/watch?v=NmqGeLNcVIg
Figure 2
Figure 3
-Truing the Rotor
If you have followed the caliper adjustment instructions and when you spin the wheel the rotor can be seen or heard intermittently contacting the brake pads, your rotor may need to be trued. The easiest way to do this is to look through the "window" of the caliper as mentioned previously and spin the wheel (keeping your fingers clear) and you can stop the wheel when the bend in the rotor comes up. (Figure 4) Note this part of the rotor and with clean hands or a rotor truing tool, apply pressure to the rotor area that was bent in the opposite direction. It does not take a lot of force to do this so start with a little then turn the wheel so the rotor passes through the caliper again to test. When done properly, the rotor will no longer be bent enough to contact the pads in the rotor and the rotor should spin quietly making no contact until the brakes are applied. (figure 5) If a rotor cannot be trued, replacement is recommended.
Below is a video showing this process:
https://www.youtube.com/watch?v=Hg36OCTAvQo
Figure 4
Figure 5