When learning how to back squat it is important to learn how to breathe properly. Focusing on taking a deep breath and resetting for every single rep will really help to improve your technique and help you to lift heavier.
The best way to do this is to scale back on weight first. Get the barbell into position on your back. Create a shelf for the bar by bringing shoulder blades back and down. This will create a stable shelf for the bar to sit on (the bar should NOT be on your neck!)
Once the bar feels comfortable and stable, take a big breath in and brace your core. When taking the big breath in, you want to really breathe into your stomach and feel your diaphragm (and ribcage) expand. Hold that breath! Now imagine someone is coming to punch you in the stomach, it is that brace position we want to hold. These actions combined, increase the pressure inside the abdominal cavity. This will increase the stability in your lower spine. As you come back to standing you may find you perform a forced grunt. This happens when we try to limit the natural desire to exhale on the way up. This forced hold is called the valsalva maneuver.
Please note that when squatting lighter weight, for higher reps this may not be needed and often breathing in on the way down and out on the way up is taught here. However it is vital that as you start to progress and squat at a heavier weight that you really create that pressure through your breathing and bracing. I often teach this as clients begin barbell squatting so that it feels much more natural as they progress with weight.
Remember to always squat safely and protect your spine!