If you are new to snorkelling, have you any idea how competent you will feel in open, deep water? We have found that theory and practise can be quite different for some people. I would start your snorkelling career by going to Brewers Bay and walking out on the the left side of the bay (under Nicole's, the cafe). Snorkelling here is pretty good and you can stay within your depth until you feel happy to venture further. Always take your time when snorkelling, float along gently, not kicking and splashing and you will see much more than you can imagine. We used to make a first snorkel stop the Indians, until I realised that some excellent swimmers but new snorkellers were not happy in open, deep water. No one admitted it at the time, but did do so after they gained confidence.
I'm afraid I don't have knowledge of which snorkel tours are good departing Tortola, but others will chime in.