How do I make sure that I don't run out of air?

Monitoring your air supply is one of the most important things you need to do as a diver.  Learning to do so and learning how to conserve air is important.  You can use less air when you stay relaxed and keep your movements slow and controlled.  Be efficient in your movements and do not over-exert yourself.  Take long, slow, deep breaths, but never hold your breath.  If you are using up your air more quickly than your dive buddy and the others you are diving with, then swimming 3 m (10 feet) high will help because you use less air at shallower depths.