It could be a number of things. Since you're a novice I'll pick on most of the 'novice' things. (They're still specific to what you describe though)
Your chest should be up and you should be squatting 'between your legs' and not on top of them.
A couple of things can screw up your squat pretty good.
First off mobility is a fancy way of saying 'able to produce a specific position'.
If your ankles aren't very mobile then you aren't capable of putting them into a position which will keep you balanced you in the bottom of a squat.
To check this try squatting with your heel raised, do this by placing something underneath your heel (something not too thick, like a 2.5lb plate), and if your squat improves you most likely need more ankle mobility.
If your hips aren't mobile then your lower back will most likely round as you descend (also called a buttwink). This is not good for your lower back or your squatting ability.
To check this you're going to want to do what is called a 'squat to stand'. If you can do it and produce a position with your chest up and back flat, your hips are doing alright. If you can't (which is most likely the case) then you need to get some more hip mobility, which can be easily done by using the squat to stand exercise or a variation of it where you grab an upright support and assume the squat stance.
Dan John does a better job than I do though, so watch this:
Also, squatrx videos (on youtube) are pretty good. There are quite a few and I can't remember which one has the mobility info so you'll have to search for that on your own.
Edit: Hamstring tightness is not even remotely at play here.