Actually it varies based on body type.
The most important things are to have a proper arch to your back, this means that your butt goes back first and then you go down, and to go slow esp at the top and bottom of the lift.
Your spine naturally curves toward you belly button, that's the arch you want to maintain.
You go slow to prevent momentum from tearing up your tendons and ligaments, a quick change of direction or stop plays hell on you joints.
Many will disagree with this, but i prefer full squats. That's where you go all the way down, a$$ to the grass. A lot of people will tell you that you are killing your knees that way, but i disagree. Doing full squats reduces the amount you can lift tremendously and will lighten the load on your joints and lower back.
Start light as you learn and get somebody that knows to watch you and correct you. Do them slow and you get stronger faster and you wont get hurt.