How do I drop some clothing sizes for prom?

really need help losing weight! I’m 15, 5ft2 and about 13stone. tried different diets, weightwatchers diet most things I ate were weightwatchers except wheatabix 4 breakfast, did that for 6weeks and only lost 4pounds but 2pounds went back on next day because apparently your weight fluctuates so I didnt lose much, then tried slimfast and that did nothing. been a comfort eater since I was 7, was bullied badly and found sweets made me happy when I felt down so I ate lots, this has been going on for 9 years and its hard to break the habit but I have began to need food a lot less because I am happy now I got friends and not bullied as much. problem is I cant excersise because it hurts the bottom of my legs, knees, hips and got a weak ankle not healed properly from a fracture 4yrs ago, plus I hate doing stuff I feel is hard like running or aerobics and stuff but I do like trampolines, dancing/dance mats and badmington which are forms of exercise but we cant afford thoses things and I’m scared to go out to play badmington because a lot of people still bully me. I need this not only to look/feel good at prom (in June 2010) but so I’m healthy again too. I’ve had a test for thyroid gland functions because my dr said that culd be stopping me losing weight and I’m awaiting results. I wanna know what pills I could take and the side effects, therapies, alternative exercise and anything else that could help please. sorry this was so long and thanks for taking time to read and help.