I use several cues when coaching the squat. In order as you approach the bar they go:
"Squeeze the bar" - grip the bar tightly and pull it into your traps.
"Chest up" - Pull your shoulders back and stick your chest out like superman.
"Big Air" - Take a deep breath into your stomach.
"Butt out / Sit back" - Engage the squat by sticking your butt out before your knees bend, and continue to sit back like you're sitting into a chair.
"Strong Knees" - Don't let your knees cave in.
"Drive through the heels" - Push up with your weight through your heels - linked to sitting back.
"Use your hips" - From half way up the emphasis is on squeezing your glutes and hips and driving them through.
If there's nothing there that helps your back flexion then you just purely aren't strong enough in your lower back and abs. The simplest fix for this is to back the weight off until you can do the movement correctly - and then treat this as your new max. Do heaps of reps with this weight using good form and your back will rapidly strengthen.
Alternatively if you can't perform the squat correctly at any weight, you may be too inflexible in your hips, glutes and hammies, causing your pelvis to tilt under.
These are all options. No-one can tell you anything definitive until they actually see you squat.