Hello I really struggled with body image in high school too. I think nearly every teenager does. Its a rite of passage. I was tall and "lanky" to start then got more curvy towards end of high school. I remember boys making comments. I was lucky that my friends were bold and bolshy and could turn any sensitive issue into something funny.
I myself am sort of where you are . I have flabby bits but getting motivated. I find that the swimming being an all over workout is good but it doesn't target my bum, tum and thighs enough. I feel brilliant after a swim. I lost weight but need toning up.
. So I think the answer is a mix of aerobic (walking, swimming , jogging, boxer cise, biking) with weights, free or machines, for specific bits. I hate my belly and bum at present