What things should I eat to gain muscle and lose fat? Will this help cellulite?

I have bad cellulite on my legs at the top under my bum. Like the hamstring area and of course my bum itself. I find it repulsive and I have never been able to wear a bathing suit because of this and I'm very self conscious even wearing shorts. I'm on my feet all day and get a lot of cardiovascular exercise, but I want to start incorporating more leg exercises, especially the hamstrings and glutes, where my main problems are. Of course I know diet is very important. I think I need to try eating more protein and vegetables. I think my diet currently is too much bread and processed carbs. I want to lose fat and gain more muscle, but I really dont know how much I should be eating and of what food groups. I know it sounds silly, but my body has really spiraled me into a depression. I've been restricting the food I eat to about 500 calories in attempt to get rid of fat. I usually eat mostly vegetables, crackers, breads, and processed, sugar-free snacks. My legs and stomach bother me the most. I'm kind of small(ish) but super jiggly and gross. I really need some advice. I dont want to get bigger in size, but Im tired of being flabby. If I have all this jiggly fat on me, I'm just going to keep dieting and making things worse. If it matters, I'm a 5'3" female and I weigh 110 pounds. In the past, I've struggled with malnutrition and not eating right. I want to be healthy and feel comfortable in myself. Someone please help me, I'm such a mess!