Diane,
First, let me say most people LOVE to snorkle. However, there are some things you may want to consider. My wife tried it for the first time on our honeymoon 25 years ago. It requires that you breathe through your mouth, not your nose, and this was something that she could not (and never has) gotten used to. Also, if you're going on a snorkling expedition, you should realize that you will be putting the mouthpiece in your mouth that was probably the day before in someone else's mouth (and has, hopefully, been cleaned to some degree). If this unnerves/bothers you may want to buy yourself some snorkling gear (mask, snorkel, and maybe fins) ahead of time (and maybe try them out in a less stressful environment to see if you like snorkling)?
You should be able to find a good snorkle at a good sporting goods store or a dive shop. Be advised, some of the snorkles have a "valve" at the top that prevents water from coming down the snorkle tube and into your mouth. This may be advantageous to a first timer. If you do go ahead and buy your own gear, find a pool near to your home and try it out. This could be a community pool, YWCA, or hey, ask a local hotel with an indoor pool if you could try snorkling there for an hour or so?
If you can get used to snorkling ahead of time, and comfortable with breathing through your mouth, you'll be a lot more comfortable on your excursion and know you won't be spending all your time trying to learn to snorkle, but actually doing it and enjoying the fish, coral, etc. Also, please remember NOT to touch or stand on the coral as this is the ecosystem for many creatures and just touching the coral destroys it. Remember, take nothing but memories and leave nothing but bubbles!!
Enjoy!!!