[Marwan]

I agree with Murat, you need to be totally relaxed, some of the things that help (in terms of technique) are like:
as you dive remove the snorkel from your mouth
as you descend keep your neck straight to maximize air flow
equipment are a plus, the right long blade fins, low volume mask, weights (you should be positive buoyant the last 10-15 feet.
Always time yourslef and aim to increase your bottom time with every go, but really the key thing is to relax and KNOW YOUR LIMITS!!! always dive with a buddy