Start off SLOW, honestly you won't be flying around doing splits and backflips anytime soon. As far as your fitness goes, work on your core strength and flexibility. So some stretches every day, and go to the gym a couple of times a week alongside your gymnastics class. There's nothing girly about it, well not really, as professional male gymnasts are sooo much stronger and fitter than most.
If you try to do too much too soon, the impact on your bones and muscles will be too much and you'll do yourself damage. Don't worry, you won't shrink. Ballet is more my thing but I also did some gymnastics alongside it as a kid, took a LONG break, tried to do it again and tore a ligament. So yeah, slowly does it!