After you've got your subwoofers picked out it's time to look into something to power them.
The first thing to consider is how many speakers will be hooked up to the amplifier and how much space you will have to work with. If you're going to be hooking up an amplifier to your mids and highs, which are generally your front and rear 2 speakers, then you might want to consider a four-channel amplifier. If you're adding subwoofers into the vehicle it can become more pricey because subwoofers generally require more power than just your mids and highs. Four channel amplifiers are not too extremely expensive because by using them for your mids and highs you won't need hundreds of watts to produce a great sound quality. Subwoofer amplifiers are usually 2-channel amps and produce a fairly good amount of power. For the more avid car enthusiast that wants to go to competitions there are even mono-block amps which are a single-channel amplifier. They generate more power and are designed specially for competitions.
Buying a car audio amplifier is just like buying any component for your vehicle, have a good idea what you're looking for and which price range fits then do some research for prices and availability in your general area.
Getting into the power scales for amplifiers
Amplifiers are usually rated by either RMS or Peak Power rating. RMS is basically the amount of power the amplifier will produce per channel consistently. Peak Power is the amount of power the amplifier is capable of producing before it cannot produce anymore.