First of all - lose the socks. Either go bare-foot, some converse all-stars, or some squat shoes.

Focus on sitting back, rather than lowering straight down. Pretend you are going to sit down in a chair behind you to keep your hips back.

Your hips will move forward when you are coming up if you try to raise your torso too soon. Raising your torso will shift the weight back, which causes your body/knees to go forward to compensate.

Focus on keeping your knees out going up and down. Knees-out is really important, so get into the habit now before the weight starts getting heavy. You'll eventually get sore/injured knees if you don't keep them out.

Best thing to do at this point, is to go to the Strengthmill.com forums and post this video in the Mark Rippetoe Q&A section. Let Rippetoe himself critique your squat, which he does very often in there. While you're at it, browse his forum and you'll see many of the common things he tells people in his critiques. You'll also notice that he says, "by the #*&king book" rather often, lol. He covers all this stuff in great detail in SS. The Squat isn't something you want to try and figure out on your own - you need to know all the little things that are required. It's worth every penny.