Where to start?

I think you've started by posting here.  Then I would take stock/gather the minimum gear you need - a good pair of running shoes, a bike, a helmet, a swim suit, and some goggles.  You don't need anything special now; but if anything I'd make sure my running shoes are right for you by getting a fitting at the local running shop.  Then I'd take a few weeks trying out each of the sports, find your starting point in terms of ability in each. Concurrently, pick a race and put together a training plan.  Not sure where you live, but depending on where you are the season ends in the next couple months; you may find your best first race is next spring.  You can search for races on this site. 

You can find training plans here, other places on the Internet, and Joel Friel books on Triathlon are great (and include training plans).  The online mentoring programs open up again (I believe) this fall.  Check them out, I will be.

And I'm sure you'll learn a ton by hanging here.

As far as losing muscle mass, I don't think training in the tri-sports will diminish them, but your expanded training focus will probably cause you to lift less.

Good luck.