When to breath:
When you are executing an exercise you want to exhale always on exertion.
What is exertion?
Exertion is the hardest point in the exercise where you need an extra "umph" to help you complete that rep.
Example: Biceps Curl
Inhale: downward phase, as you extend the arm and lower the weight
Exhale: upward phase, as you flex the arm and draw the weight up to shoulder height
Try using this breathing technique, and then try it where you hold your breath where you should be exhaling. You will find that by exhaling on exertion allows for the exercise to feel easier, and for you to progress to heavier weight.