Getting in shape for track?

I'm 5'5 and 110-115 lbs. My time for the 100 meter dash was about a 13:01 when I last got timed and I'm pretty sure I've gotten A LOT slower. I haven't done any physical activity since about March '09 and all I've done since then is eat a lot of junk food. Track is starting soon and I'm a 9th grader in high school if that makes any differece. I'm not in athletics this year but I am in a dance cl, but I doubt that has really helped my physically. Any ideas?