The best advice I can give you is to start with a easy site like Mexico Rocks/ Tres cocos where the water is very shallow & not much current, after that try Hol Chan where it's deeper & much more fish, the current at Hol Chan is stronger because it is shaped like a horse shoe & very close to the barrier reef.
I had 3 gals who never snorkled before & it only took them about 10 min. to catch on at Mexico Rocks, we went out with El Gato.
Practicing out by one of the piers is a good ideal also.
Just relax & enjoy!!