First of all, check what kind of squat rack your gym has. In commercial gyms, the squat racks are often U-shaped frames, with two safety bars either side and a tall rail of hooks in the middle.

How to set up a squat rack

Place the barbell on the hook that’s just below shoulder height.
Stand under the barbell so it is resting on your upper back with your feet shoulder width apart. Your knees should be slightly bent to fit under the bar.
Straighten your legs so you’re fully standing, lifting the barbell off of the hook as you do so.
Step backwards so the barbell comes away from the hooks – you’re now ready to squat.
When you’ve finished your set, step forwards so the barbell rests against the frame.
Bend your knees to place it back on the hook.