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.

Beginner: Dumbbell squats or bodyweight-10lbs plates on the machine
Intermediate: 25-55lb plates on each side
Advanced: 60-100+ lb plates on each side

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