I am 14 years old and I want to lose weight.?

In school we measured our BMI and mine was 19.something. I know that is healthy, yet i have a flabby belly and FAT legs. I HATE it! I want to go on a diet and tone my body a bit but everyone tells me not to? ! my whole body feels like it wobbles when i walk and i feel sick. I walk about 3 miles 5 days a week. but i never lose weight? !! what can i do to get healthy/ toned? is it excersise i need ? or diet? helppp!