I just read another post of yours, and you say you're 5'5, you are
NOT fat.
Here's a link to a healthy weight range chart, which shows you that you're smack bang in the middle of being a healthy weight for your height.
You mentioned in the other post that your mum tells you that you're fat, and that you used to have bulemia. Ignore your mother. I don't know why she tries to sabotage you, but you have to ignore her. Is there any way for you to buy your own food? Or would your mum get you fruit and veggies if you asked her? You also mentioned that you see a therapist, so she should be able to help you. Get other family members, or teachers to help you out if it dosen't get through to her.
Self esteem is harder. Your therapist should be able to help you with some ways with that. I was a little older than you when I saw a therapist, but it helped me a lot, so I hope you have the same outcome.
One thing I did as a teen, was write down every compliment that was given to me, and read over them whenever I felt bad. It does help. Look in the mirror and tell yourself something you like about yourself every day. It might be only little at first (eg I like my eyes) but you'll soon get better at it.
Unless you're 4 foot, try not to focus on losing weight, but being healthy. Drink more water, eat more fruit and veggies, go for a walk every day. I don't know if you play sport, but if you don't look into playing a sport with your friends. Social, fitness and fun all in one!
Remember, you are NOT fat.