For most people "thinking about breathing" is not something you ever consider. It's an automatic physiological action, and at best if you're someone that meditates you may spend some time paying attention to it. When it comes to running, breath is something that you might start to dial into, especially when you're out of it.... keeled over, holding your knees after a hard effort.
As a running coach I find that "How do I breathe"? is not usually the first thing on a list of questions my athletes bring, but its one that inevitably comes up. The question is usually seeking to solve the problem of how they can better control their breathing, in hopes to perform in a run.
While developing Aerobic fitness is a huge factor in how hard someone needs to breath, just like your stride, there are techniques that enable greater efficiency and control. As I mentioned above in point 2, most of us breath very shallow breaths as we go about our day to day.
Sitting at your desk or kicking back with Netflix requires much less oxygen, so naturally we've conditioned ourselves through this automatic process to take in the minimal amount of air required to support the physical demand being placed on the body at any given time. The challenge that this creates as a result, is that when our body really needs more oxygen (like in a run), we're not that effective at breathing with intention.
The good news is that it's something that you can improve, which will translate into a better run and potential for reduced injury. Below are tips to consider and methods you can start to implement right away.