your going to want to find out if you have a percula or ocellaris clown. BOTH can look exactly like nemo and you want to accurately identify which you have or else the fish def. wont breed and are almost guarenteed to fight/kill eachother.
the ultimate best bet would to be to just buy an actual breeding pair from the start and sell/trade in the fish you have now. be careful if your doing this as people sell these fish as "PAIRS" and as "BREEDING PAIRS" as it isnt hard to get 2 of these fish to pair up but can take some time to get them to breed while paired up. If looking to buy a breeding pair of clowns, ask to see their eggs as proof of them being in breeding condition.
If you buy two small clowns of the same breed then eventually the larger, more dominant clown will transform into a female. Your then going to have to wait until your clowns grow and sexually mature. Your going to want to offer a very large mixture of foods and live foods from time to time.
Usually clowns will breed on their own in tanks but the other fish usually eat all the babies or they get sucked into powerheads and skimmers. For this reason if you want a max yield, setting up a fry tank may be best with a sponge filter.