I can't seem to do a squat without lifting my heels.?

Whenever I attempt a squat, the only way I can seem to do it is on my tip-toes! I've watched videos on youtube and everything and I don't understand how you keep your whole foot on the floor. Do you get what I'm saying? Please help!
Update: And to be sure I'm doing it right, are you supposed to keep your back completely vertical?
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