One of the most common mistakes coaches see in novice and intermediate swimmers is their breathing. Being able to breath correctly will make freestyle swimming much easier. It will improve your balance in the water and surprisingly will make you a lot more relaxed in the water.
The secret here is to become more relaxed at exhaling and to do this whenever your head is in the water. That way you only have to breath in when you rotate your head to breath.
Most of the time swimmers can be seen to be exhaling just before they turn their head to breath, getting ready for the inhale when the head is turned. Exhaling whilst the head is in the water has several effects.