% Weight
You should basically eat calories that are nutritiously dense, and get plenty of exercise. Exercise just means that you are physically active, so for instance walking around a lot or riding a bicycle to work/school can be exercise. Try to eat a balanced diet of vegetables, meats (unless you're vegetarian or vegan), carbs (potatoes, rice, bread, etc), with plenty of protein (meat, lentils, nuts, broccoli, eggs, etc). Don't bother with diets, they don't really work and they're never long-term even if they do.
For people who are morbidly obese, liposuction is a legitimate way to deal with weight loss.
http://www.nhs.uk/conditions/Eating-disorders/Pages/Introduction.aspx
Although they are not specifically oriented towards supporting Transgender people, BEAT is the UK's biggest eating disorder charity/support service. They can also assist in finding treatment.
Youthline: 0808 801 0711
Helpline: 0808 801 0677