You have your Sub ready, you have your head unit sorted, now its time to look at your amplifier (amp).
When sorting out your in car entertainment system all 3 components should be carefully taken into account. Overloading your system or draining your battery would not be fun. Or cheap (if it goes horribly wrong). The role of the amp is to boost the power to your car speakers and subwoofer so that you can utilise the full sound quality of your system.
The amp works on the battery power of your car. If you want to get into some serious power it could be worth putting an additional battery in to feed your systems energy requirements. If not just make sure at least your current battery and alternator are in good working order until you want to upgrade.
When you are installing the amp, make sure that you look check that the speakers and sub are compatible with the amp you choose or you will get a redundant sound.