If you installed an aftermarket amp, does the power light on the amp come on? If not check the the amp as it should have its own fuses (read the manual) check them. If those are good, then check the fuse on the power wire to the amp, should be close to the battery if installed right. Also check the ground wire of the amp. If you have a Multimeter, you can start to take reading form the AMP power wires and follow the wires back and see where the power starts.
If the amp is getting power, check the speaker wires and speakers. You can take a 9v battery and touch the two speaker wires, one each side, and the speaker should make a small pop sound, if not, the speakers are blown.
~K