Undecided Question

so i used to weigh like 199 in the beginning of 2009 and now i weigh 153. no diet or anything idk how it happened and i dont like the gym or anything so i kinda have loose skin now on my legs and belly. if i suck my stomach in you cant tell but if i lay down on my side you can. im 18 and a female. i dont eat the healthiest but i loose my appetite during winter which is the reason. how do i tone my stomach and legs. and i have stretch marks on my legs hips and sides. how can i make them go away? my boyfriend dont seem to care bout the body but i feel i would be more comfortable during sex and changing in front of him