First food of clowns is rotifer. You need to feed them rotofers for about at least ten days and then convert to dry powder foods, either Golden Pearls or Otohime. Rotofers need phytoplankton as food. You can order frozen phytoplankton and rotifer start culture from Reed Marineculture. You can also order Otohime food there also. You don't need to order rotifer if you can get a small sample of it from someone who breed clown fish.
The specific of how to do these culture you will have to read either online or buy a book