Almost every new diver surfaces first and quietly assumes something is wrong with them. Nothing is. Air consumption drops by roughly a third over a diver's first twenty dives, without any deliberate effort - but three things make it happen faster.
Sort your weighting
Over-weighted divers add air to a BCD to compensate, swim slightly head-up to stay level, and burn gas fighting their own trim. A proper weight check on dive three is worth more than any breathing technique.
Slow down
Drag rises with the square of speed. Swimming at half the pace does not cost you half the air - it costs you a quarter of it. The divers who stay down longest are always the ones who look like they are barely moving.
Stop using your hands
Hands do nothing underwater except burn oxygen. Fold them, or hold your camera with them, and let your legs do the work.
What not to do
Skip breathing - deliberately holding your breath to save gas - is dangerous and does not work. It raises CO2, which makes you feel breathless, which makes you breathe harder. Breathe normally and deeply.