You need to find an amp that has the amount of RMS power at the ohm load you will need for your speakers. Choose your amp and speakers together, to work well together.
If, for example, you want to drive a single 400 watt 4 ohm sub, then you need an amp that will give you 400 watts RMS @ 4 ohms. Not one that says 400 RMS watts @ 2 ohms; that will probably only be 200 watts @ 4 ohms, and not one that says 400 watts peak @ 4 ohms.
If you have a small alternator in your car, but want a big sub amplifier, then you should look for a class D amp, which is more efficient than a class A/B amp for the same amount of current.
So a 400 watt class D amplifier may only draw 30 amps of power, but a 400 watt class A/B amp may need 60 amps of power to produce the same watts.
Stick with companies that have a good reputation for making amplifiers and avoid the cheaper off brands with lots of chrome and fancy lights and displays. In a cheap amp with chrome and digital displays, that's what your paying for, not quality internal design and contruction.
Tell us what your trying to do, and we can give you specific recommendations.