First, it depends what kind of kick. Before you even consider being able to do it powerfully, you have to learn the technique.
Then, it takes alot more practice to make it decently powerful. I'd suggest kicking pads and/or a heavy bag, then being able to reliably break boards.
After you have at least decent muscle memory w/ the kick and are capable of using it at least somewhat forcefully, then comes the hard part; Learning to connect with it effectively on an opponent, let alone in a self defense situation.
After you've done all that (probably a minimum of a few years of training), then you can start to worry about nailing people in the head with kicks in a self-defense situation (a challenge for even advanced martial artists).