Step 1- Determine the job the amp will perform.
Choose the right amp for the job. If you want to power 2 speakers buy a 2 channel amp, if you want to power 2 speakers and a sub, buy a 4-channel amp, you can also power 4 speakers and a sub with a 5 or 6 channel amp. If you are using your amp strictly for bass, consider a mono subwoofer amp. The single-channel amps are designed specifically for low frequencies with specific controls for maximizing bass output.

Step 2- Choose the features you'll need.
Subwoofer amps- amps with bass boost circuit and a built-in low-pass filter are good to have on an amp driving a sub. Bass boost provides increased deep bass impact. A low-pass filter sends only bass notes to your subwoofer.

4-Channel Amps- A built-in high-pass filter relieves your smaller speakers of the burden of tyring to reproduce low bass that they aren't designed to handle. With this filter your speakers can play louder with less distortion.

Factory Radio- If you just have a factory radio, make sure the amp has speaker-level inputs. Speaker-level inputs also allow you to use the factory speaker wires to send the signal into the amp.