I can't squat.

Sorry if this is in the FAQ, I looked but didn't find anything.

Title explains it all: I can't squat properly. I'm working on it, but it's frustrating and I don't think it's getting any better. I think my problem is flexibility: when I get low enough, I either have to stick my butt out too far or I fall backward. I also feel a lot of stress in the ligaments at the front of my ankle/foot. Are there stretches I can do to fix this? Exercises? Someone suggested using a door frame, but I don't know what I'm supposed to be doing, exactly.

Any help is appreciated!