Swimming is unique among the endurance sports because you can only breathe at certain intervals. In the freestyle, you breathe at the end of each stroke by rotating your head until your mouth is out the water. The problem is that you are only in that position for a moment and it may feel like you don't have enough time to get in enough air.
One solution is to breathe out slowly while while your head is rotating, then when your mouth finally comes out of the water to begin breathing in. I used to wait until my head was out of the water to breath out and then in, but that cuts down your already short window. You can breathe out anytime, but you can only breathe in when your head is out of the water.
Another tip is to try rotating your head to the side and back, like you are moving your chin to your shoulder. There is a little more space there to breathe, because your body makes a wake in the water and the further back your mouth is the lower the water level.