Form......
Get the bar low on your shoulders. I try to place the bar under my traps where they connect to my delts. The back of my delts make a nice indent like a shelf to set the bar on. Place your feet shoulder with or a little inside of that apart. I point my toes slightly out. Keep your back strait. I like to look strait forward or up a little to help me keep my back strait. Squat down keeping your shins strait knees over the heel of your foot. If your knees get over the ball of your foot you will be off balance and have to use your hip flexors and gluts to help push the wight up to much. Squat down to parallel or below keeping back strait and knees over your heels get to bottom position and push up. Remember to look forward or slightly up to help keep your balance. Practice good form and the weight will follow.

Hope this makes sense for you.

saturn somes it up, alot of people new to squats try to do high bar squats, but you want it low, low bar squats give you a better center of gravity, less room for injury. Make sure you come down all the way, eep chest out shoulders back, dont arch back and puch with legs. go parrallel. i go further than that image, but try it on a smith machine, do some box squats

I have been doing smith machine squats for the past 5 months, then the other day I decided I was done with those, so I hopped in the rack and had to take a damn decrease in weight. But as long as form is good that is ok. I enjoy the free squats much better.