You really need to stop over thinking it, get in a pool and start doing some lengths. Take regular breaks at first and and build up the distance you take between breaks.
Practise breathing on both sides.
When you are a bit more comfortable in the water think about a lesson, my mate just did one with video capture and coaching, he said it was excellent.
If the swim is open water make sure you schedule in some open water sessions in a wetsuit (if it is needed for your event).
Apparently over half of first timers at triathlon swim breaststroke, so don’t worry too much about that. It’s always there to fall back on.
Oh, and ask on a triathlon forum. You’ll get better advice there than on a mountain bike forum.
Swimming is mostly technique, so whilst your fitness won’t hurt, it won’t help much either.