Squats are a great workout, but they do *not* truly work the back. They are primarily working your upper legs, though they are a "whole body workout" in a sense.
Deadlifts. Put weight on a bar on the floor. Bend down, keeping your back straight, looking forward, and grab the bar with a supinated grip (your strong arm palm facing inward, the weaker arm palm facing outward). Try to lift with your legs, pushing your hips upward while keeping your back firmly (but not excessively) straight. Pull the bar up keeping it close to your legs, and it should nearly come in contact with your knees. Stop when you are standing straight up and holding the bar across your thighs.
Repeat. Start with a bar only and work on form, watching videos of people who know how to do it right.