How do you register for selective service (the theoretical draft)?

I'm an 18 year old male, and I've heard that you have to register for selective service and that you automatically are registered when you register to vote. However, I am going off to college in a few months and want to register to vote at the city where I am going to college. Is it necessary that I register ASAP, or since there is no impending draft, is it legal for me to wait until August or September?