Idaho gave very good advise. I would only add a couple of things to what s/he had to say.
I too was very claustrophobic. Back in the '60's I would panic when the mask sucked onto my face. So I stuck to wading for abalone.
The equipment has greatly improved. When I learned that you can actually breath OUT through a mask with a purge valve it opened a whole new world to me.
My friends took me to their private pool, let me sit on the steps and put the mask on my face. I could breath in through my mouth and out through my nose. I did this for a while. Then I put the snorkel in my mouth and swam around for a while breathing in and out through the snorkel.
Next I put the regulator in my mouth and went through the same routine: sit, then swim.
It didn't take long for my brain to accept that I would not suffocate.
I am now an avid diver and so happy I didn't miss this opportunity.
Take your time and be with people you really trust.
Of course, if you decide not to dive it just leaves more ocean for the rest of us.
AmbergrisCaye.com 
