How can I motivate myself to work out and lose lots of weight?

I have had two children and am normally super super skinny, like a size 2 for a 5'7" height, but now I'm a size 12/13-14!! I feel so gross and fat and everyone who knew me before are like shocked to see me like this. I never had to worry about my weight before. I've had my gall bladder taken out with this last pregnancy and since then I've just been ballooning up and I'm NEVER FULL!!! HELP!!