Find a weight that challenges you, but allows you to keep proper form. Engage your core, slowly drop into a squat, keeping your core engaged & push through your heels. Make sure not to fully lock out your legs.

Weight Recommendations:

Beginner: 10-15lbs plates on each side
Intermediate: 25-45lb plates on each side
Advanced: 50-90+ lb plates on each side of the barbell

Modification: you can do regular standing squats with any type of weight