Hello asan, welcome to DB.
Try reading through this:
http://forums.deeperblue.net/beginner-freediving/64959-how-start-freediving.html
It may seem long, or take a while, but the tips will get you off to a great start.
In general before a dive, for at least 2 or 3 minutes, try to be totally relaxed and floppy on the surface, not a single stress or effort in a single muscle, jaw, neck etc, all 100% relaxed.
Whilst relaxing (close your eyes maybe) take normal breaths, just like you were sitting watching TV, normal frequency and depth. It is tempting to accelerate the frequency and depth, but you will be hyperventilating which is a big NO NO, and could kill you (read why in the above thread).
Just before you dive, I recommend you equalize your ears and mask, take two full breaths, fastish in slow out, then the last full breath, starting from your diaphragm, then your chest, and if you like your neck/shoulders.
And off you go. Relax, enjoy the dive, and come up when your body says so (this point takes years to learn, and many never find it precisely).
Dive with a buddy, take it in turns, one watching one diving, more fun and the only way to be safe.
Cheers
