Thinking about getting into Marine

-how many pounds of live rock will be needed for a 37 gallon tall tank?
Gerneally, you want 1.5-2 pounds per gallon. You can reduce cost by using lace rock (or other marine safe porous rock) instead for a portion of this--it works just as well, and costs much less. Within 6-18 months, you won't be able to tell it apart from the 'real' stuff.
what stocking levels are realistic? I was thinking about 2 firefish, one royal gramma and two percula clown fish or banghai cardinals
That would work. It would be tight, and you may have problems with the firefish--they can be sensitive and prone to jumping, so a well covered tank is a requirement.

will I need any additional equiptment other than a power head, heater, protien skimmer and air pump?
Maybe 2 powerheads. With the rock and sand, the filtration is taken care of, and you'll be able to add a good cleaner crew for mechanical cleanup. Of course, a hydrometer or refractometer is needed, as well as buckets and such for mixing the saltwater. Depending on your water supply, you may need to use filtered water, as well.

Not really. But, in a smaller tank, fewer fish will be better anyway.