Practice on land. If you go for a jog hold your breath to train your body. Go for say 10 light poles then hold your breath till the 11th. Do that for a while and then increase it to every 9th pole. SLOWLY work you way up and then start going from say the 10th to the 12th pole holding your breath. Over time your body will learn to use less oxygen and use what it does better which will result in you going deeper and longer in the water. Time in the water helps too. Just what ever you do take it slow and steady and dont push yourself. Shallow water blackout wont kill you but your first breath underwater could.

Matt