(Im a 15 year old female) Well whenever l do sit ups, l do them so easily it feels like im not building any muscle at all..

No im not fat, I am average body weight and l would like to tone my stomach/legs/arms. Ive been trying to for about a month or so.

Any ideas of a good exercise?

wii fit

if you can, get a bender ball.
im guessing its available on the internet.

Try with the exercise ball - its more challenging and will tone up fast