I need to lose weight

I'm feeling very down lately, I haven't felt myself in a long time. I have lost all motivation to do anything and I wondered if any of you have any advice? I need to lose weight because I'm the biggest I have ever been and I have never felt so rubbish about myself, but I don't have the motivation in me to stop eating crap and get off my ass and exercise. So yes, it's my fault that I am this way but I just can't help myself sometimes, I see all these other's girls, even including my friends who are much thinner then me (they're like a size 6 or 8 in uk size and I'm about a size 12-14) I feel awful when I'm stood next to them all, I just feel like an elephant. And then they start complaining that they're getting fat and it just makes me feel even worse then I already do, because I'm double their size, I just want to cry about it all the time. I've just gone into year 12 at school and loads of girls from different schools have joined and I'm just so much bigger then them all. It's really getting me down and I do need to do something about it, so does anyone have any tips to help me lose weight quickly? And to help me stop myself from temptations??