I wrote an FAQ on clams, it's stuck at the top of the Clam Fourm, and that should give you some background info and examples of different clams to start from
Basically you can get whatever you want I think, I always reccomend starting with a derassa though, it's a sand clam, and they grow quick. They are not really easer to care for, but you tend to get them bigger, so they can take more abuse, and there not as prone to pinched mantle so that's helpfull. They also don't have scoots for prymid snails to hide in which makes finding those easer.
HTH,
Whiskey
