How do you be a hunter in left 4 dead

All you have to do is play online. Then, you just play and wait your turn. You will be a Smoker, Hunter, Boomer, or, at some points of time, you will become the tank. There is usually one Smoker, One Boomer, and two Hunters each round. Once that round is over, you will no longer be the infected and you will have to wait for the next round.