Try to run at a pace at which you can breathe easily. Use the "talk test" to figure out if your pace is appropriate. You should be able to speak in full sentences, without gasping for air. If you can't do that, you may have to slow down or take a walking break. As you become more fit, your breathing will become easier.
If you find that you're wheezing and have trouble catching your breath even when you slow down, talk to your doctor about whether you might have asthma.
Video: Don'ts for Breathing When Running