Hey Gillian its me again. It really does suck that your family is not supportive of you and your attempts at being healthy. The best you can do in your situation is what others here have already said. Portion control, since you are a teenager you pretty much don't have control over what you get to eat at home, and that blows. The only thing you can do in this situation is try and eat less. It will be hard you will feel like you are still hungry, but over time your body will get used to this. Humans were designed to eat as much as they can when they can, and this is the downfall of many people in a modern society where food is never scarce. Another thing you can do, and this is a hard one (I am currently trying) is too only drink water. Cut all soda and juices, soda is obvious why, juices people don't understand why they think "but it has vitamins its good for you", the problem with that is that juices have no fiber to help absorb the sugar in the in the juice. Go ahead check how much sugar juice has, it can be up to as much as soda if its not natural juice. Tea is okay to drink as long as you make it or its sugarless tea.
Now on to exercise. That is horrible that your family makes fun of you for working out. I good possibility as to why they do this is because they don't want to see you succeed. Especially if they are overweight as well. It brings up their own insecurities about their weight. It would be hard to fight through their comments when you workout but if you can do it it will help you a lot. This is not only because you will losing some weight, but mostly because you can say that you are doing it despite what others say, you are taking care of you. This is very important in building self esteem. Also as someone else mentioned, walk. Just walk 30-minutes as many days as you want to a week. Even if its only 1 day a week, that is something.
Gillian, something else you need to know, something that lead others to just giving up. Weight loss can not be achieved in a short amount of time healthily. It is something that happens over months even years for some. It takes true determination, and work. You can not put yourself down one month down the road because you only lost 1 or 2 pounds. For some people thats not only is losing 1 or 2 pounds but also not gaining 2 or even 5 pounds that month. That is something amazing.
You know how to lose weight even with the people around you. Its just very difficult. I would like you to visit this online community http://www.reddit.com/r/loseit . take a look around read what others say. Then maybe you can join in. Introduce yourself, there are many people there that can help you and will support you throughout your journey.
I wish you only the best Gillian.