1) Make sure your tank is really mature and stable! If you can keep sps, you can likely keep a clam
2) Research the type you want. Some are rock types, some are sand types. Some like high light, some don't like AS high of light
3) make sure you don't have any clam eating fish or inverts in the tank (more research!)
Clams are so fun! They just really need light and stability to thrive. Oh, and they will demolish your calcium levels. They use a lot of it (relative to size and type)