Disc brake maintenance
There are a number of simple things you can do to try improve performance before replacing the brake pads.


Check the pads to see how worn they are. Here is an example of a new and worn pad requiring replacing.




If the pads don't appear to need replacing you can:


Lightly sand the pad with fine sand paper
Clean the rotor with isopropyl alcohol (or any non-residue solvent)


If that eliminates the squeaking but doesn't improve performance the next step is to bleed brakes. If you have Avid similar brakes the process (similar for Juicy and Elixirs) is more involved than the Shimano process and make require professional assistance.