on the off chance you don't understand the "pushing too big a gear" suggestions, I'll chime in simply...
What gearing are you using on your rides? If you are locked in the big chain ring (up front) and are using lots of muscle to power the pedals around then you are likely in too high a gear. Shift to a lower gear so it is easier to spin the pedals around even if that means using the smaller chain ring up front and/or the bigger cogs in the back. It may seem "too easy" but the the easier (and faster - "cadence") you spin the pedals around the more you will use your heart and lungs for aerobic training rather than muscling through anaerobically. Doing the latter will leave your muscles zapped but do little for you aerobically which is really not what you want. Keep it easy to spin and give yourself the aerobic workout you want and need.
G'luck and welcome to cycling!