I would like to lose weight, have lack of confidence

Ok, I am 17 yrs old, 5'5 and 204.0 lbs. At first this year I had decided to go out and exercize (I havn't in a long time which was stupid of me). I'm not into sports (no offence sport ppls), but I love going on walks. So I started walking. At first I enjoyed it, I was thrilled. But now, I don't really want to go, and I haven't a clue why. I'm so into self-image now, that I litterally get depressed (which makes me want to eat). Along with that, I am fearing Thanksgiving, and Christmas since it's the "Eat and be Merry" season. o.O I feel bad because I am semi-short, but really over weight. I have heard stuff said behind my back about my weight, even by my brothers. Don't get me wrong, I love food, but sometimes (after I get out of depression) I don't want to eat. Then I go through long periods where I don't eat at all. Not only is this hurting myself, but I feel highly unatractive (I have a slight double chin). I have NEVER at all told my parents about any of this. Whats worse is the story of my brother (15). He was a little chunky when he was younger, but as time passed he had a growth spurt, and is now taller and skinnier then my other brother (19) who is like 6'3. My cousin also(she had anorexia) is quite skinny, and I covet (even though I know not too) her physique. I feel mentally ill and physically ill, sometimes I randomly burst into tears at my house (lukly no one has seen yet). Please, I would love some help. I do like to swim, but does it help one said lose weight? I would like to build up musule and be able to feel comfortable with myself and not be so conserned about what people think. I would like to lose weight before the coming summer, but I feel the lack of confidence. :'(