You seem to be very concerned about the sizing. If you purchased the bike at a shop - they hopefully measured you to assure the sizing was proper. 13 is a small frame mountain bike. For instance I ride a 17 - 18 and am 5'8". 15 would be for around 5'6" and 13 for 5'4" or less. Does that seem right to you?

The GT bike is a good bike. The best way to get riding is to just get out there and ride. It's fun to plan a course. Say a 5 mile out and back and see how you improve first just making the distance and next improving your time. Remember trails is tougher than paved road. Also remember tire pressure is also different for each. Don't forget to properly inflate your tires for each instance. That is the best way to prevent flatting. If you are riding on a 2" knobby, tire pressure should be around 60 for the paved road and 30 - 40 for trail. Also learned how to do some simple maintenance - like how to lube your chain and keep the bike clean.

Anyway just go out and have fun!!!