I'm sorry to say SH didn't quite hit the nail on the head. Clown fish can only change sex twice, they start out sexless and change to male, then they change to female if they become dominant, if you go with two very small fish in a year or two you will have a pair, unfortunately if you go with the one fish much larger than the other method which is much easiers and the smaller fish is still good sized you run the risk of having gotten a smaller fish that has already turned femal, in addition to that the larger fish have a bit of a temper when it comes to seeing there own likeness and you could end up with the smaller fish being harassed, best to go with two very small clowns in the begining, since I think I can assume your tank isn't big enought for two territorys.