If you can't fully squat with your body weight, you might have really tight hip flexors. Stretch them.
You mean Quad's or hamstrings, Hip flexors do not limit the range of motion in a squat. The can only limit extension.
Tight quadriceps can cause increased pressure in the knee, forcing you to adjust your squat position. Usually clients lean more forward to keep their balance causing their back to round.
Tight hamstring can pull on the pelvis causing a rounded back as well.
I am guessing that its your hamstrings.
Feet position should be shoulder wide apart, feet pointed slightly outwards.
In contrary to what many instructors will teach you, you are allowed to move your knees over your (second and third) toes.
Like said before, push your butt back,keep your chest up and don't look down.
If your still having difficulties try to do a swissbal squat (the swissbal between your back and the wall) or place small weight under your heels so your heels are slightly elevated. You can also hold a dumbbell in front of you between your legs to keep you in balance.
but most importantly stretch those hammy's