Hi there, I'm doing my first triathlon around the same time you are. I think that we're in just about the same situation. Last year in high school I was a runner. I run all the time still. I'm not worried about that. I picked up road biking over the summer and enjoy the hell out of that. I have a common knowledge of how to swim, but I too have no idea how I am going to conquer a half mile ocean swim. I'm signing up for the pool as soon as I can. But I have heard from many people that swimming in a pool and swimming in the ocean (not sure if thats what you're doing) are two differnent things. Also I understand swimming with 50+ people all around you can be a bit overwhelming.
Train hard. I think it will pay off in the end. Best of luck to you.
-Dan-