How do i get a toned belly afer being so ill? please help?

hello in tasha 17 and female. ok so i have had cancer for about 5 years, and now its finaly gone HOORAY! but in that time ive done quite honestly no exercise because i have been so ill, i use to be a dancer and do lots and i had a six pack now its all gone i dont really want to go back to dancing though im to tall and my legs are really long :( :( although ive done nothing im still very very thin because i was ill. my belly isnt "fat" but i do seem to have this little falbby bit just under my belly button and you can tell its so untoned. i am generally very unfit and its getting me down :) what i would like to know is how do i get my body back?? and how long will it take to get a toned belly if i do all the right things?? also how do i tone my bottom? :D