How can i exercise without losing Weight ?

Hi Im a 18 year old Female.. im 154cm tall and about 84 pounds... which means im pretty thin! ive been wanting to do some exercise to tone up my body, flatten my stomatch and improve my fitness level...Plus ive discovered that exercising really helps when im feeling a little down !!   ive been walking the track lately (over 11 laps a few times a week) but im afraid ill lose weight !! i was just hoping for a few tips on which kind of outdoor exercises i should do and what kind of foods i should be eating...when it comes to exercise my IQ is ZERO(im such a couch potatoe !!) so i really need YOUR HELP !!