Many people recommend doing a water change after the first month or two of ownership because of this problem.
AFAIK foam is a bunch of small icky stuff, like skin oils, bathing suit detergents, dead skin cells, dead stuff that your sanitizer killed, shampoo and dandruff and, well, one could go on...
If you've never owned a tub before, or haven't used it regularly, just getting into the tub adds lots of skin cells and other body gunk which comes off of you in the tub. If you don't do a second rinse of your bathing suits, it is detergent too.
I've heard that if you scoop the foam off with a plate and it keeps coming back, it may be water chemistry. Some people scoop it off and it doesn't come back.
But if it isn't too inconvienent, you may want to try a water change.
-Ed