I am thin, and baggies fall off me, but I get too much crap for wearing Lycra so baggies it is for MTBing. I wear lycra on the road, of course.
Have fun, don't try going clipless until you are a little more comfortable, then make sure you adjust your clipless so that you can get in and out much easier than on a road bike.
If you ride twisty popular out and backs make sure you have a little bell. I broke mine in a fall and now have to yell, "CA-CAW, CA-CAW" when I go around a blind turn. We have a pretty popular (and crowded) trail that attracts hikers, bikers of all levels due to the technical bits, and new trail parts that result in general mayhem. The bell really helps.
And go Camelbak. Most MTB frames only allow for one bottle at best.