Go try it and don't be one of those people that make it as you said "a life changing decision" if you go in with that attitude you will either get frustrated and quit or absorb yourself in it all the time get burnt out and quit. Take it for what it is and what your body can handle.
Go try it, do your best and for god's sake have fun. Odds are you aren't going to be a world champion based on the background info you gave but you can definitely have fun, get in shape make some good friends.
As for getting in shape it has already been said but in another light, if you want to get in shape for a marathon-run, if you want to get in shape for a triathlon- swim, bike and run, if you want to get in shape for a grappling sport- grapple. Don't be the guy that think thinks 40 minutes a day on an elliptical machine will get you ready. Odds are you will be training in an inefficient manner using either programs from muscle mags or old wives tales from people that will give you unsubstantiated "gym wisdom" that won't be the most beneficial training.