Ken,
It sounds like you have enough gear to start. Get on that mountain bike and ride. Every day. Take it to the grocery store, to work, wherever. Ride it enough that you feel like a kid again. Enjoy going fast and maybe even work on your wheelies.
Ride it to a pool and practice swimming. If you're like most of us, your swimming will really stink at first. I can ride a hundred miles, but I feel like I'm going to die after a few hundred yards in the pool. If you feel that way too, get a book called
Total Immersion and take your time learning how to swim relaxed. Many folks have promised that once you get the hang of it, you'll be able to swim for miles.
Since you have a bike and I'm assuming something to wear in the pool, the next purchase should be a pair of running shoes. Don't buy these on your own, and don't skimp here. A good pair could cost $80-100! If you don't have that kind of coin... stash away some money for a few months while you focus on riding and swimming. Once you have the cash, go to a running store like Sole Sports or Road Runners. Explain that you need a new pair of shoes and aren't sure where to start. They will ask you run for them so that they can analyze your gait. Then, they'll help you find just the right pair of shoes.
(Believe it or not, all of those different shoes really have specific purposes
). Don't shy away from this step. Don't go get any ol' pair of shoes from the mall. If you get a pair that is specifically designed for a light-weight under-pronator and you're a medium-built over-pronator, then you will start getting injuries in no time. Really. Go to a running store and have them help you.
If you're hooked on swim-bike-run by the end of summer, then sign up for the Pabst Blue Ribbon Off-Road Triathlon. It's short, requires a mountain bike and ends with a slip and slide! Here's a link to the website for more information...
http://redrockco.com/pbr-road-triathlon I wish you the best of luck and I hope you come to love triathlon.
Scott