||When your brake pads have been fully used, or when they have been worn very unevenly, applying the brakes will make squeaking noises. Doing this for too long will damage your rim (or your rotor, for disc brakes).
||Are your shifters and brake levers hard to use? Brakes and derailleurs use a metal cable that slides into plastic housing to transfer motion from the lever to the brake or derailleur unit. When that cable gets old or rusted, the cable slides poorly in the housing, and motion is not properly transferred from the levers. Keep your bike out of the rain to prevent this.
||Most brakes need to be tightened and centered periodically to make sure they are not too loose or coming into contact with the rim (or the rotor for disc brakes) when you are not braking. This makes pedaling a challenge, and can quickly wear down your brake pads or damage your brakes.
||Your Front and Rear Wheels, as well as your bottom bracket (where the crank meets the frame), and your headset (inside the front tube of your frame) all turn by a ball and bearing mechanism. If either of them feel tight, very loose, or make clicking noises when turning, they probably need to be adjusted. Lower end and older bike models often have “unsealed” bearings which required proper pressure to turn smoothly. They can usually be adjusted, but if you leave it unattended for too long, they will break and require costly repairs.