I went back to school around your age - it was awesome!!! I found that due to my age, I was a more responsible student than the 18 year olds were. They would forget to do assignments/forget about exams etc and I kept lists/prioritized etc. I didn't get the awesomest grades though, I stupidly took classes that were too hard (Physical Anthropology) (Latin - eeewww) and (Sociology OMG) but I passed every class and never dropped any classes. My worst mark I think was a C- in LAtin and I worked my hiney off for that C-!!! :O
I would start off with intro classes and do NOT take too many classes right out of the gate. Go easy on yourself. Make time to study and study a LOT. Form study groups. AND make friends!!!
[Spock]I have always been, and always shall be, your friend.[/Spock]