Most people can hold their breath longer than they think.  If you follow a few basic steps you will probably be surprised of
what you are capable of.  First however, you must have the basics down.  You will need a pair of fins and a quality mask and snorkel.
(No cheep $20 set).  A scuba shop is a good place to purchase quality snorkeling equipment.
You should be comfortable enough using your gear that snorkeling becomes second nature.  If you have troubles with a leaky
mask or snorkel or feel like you are struggling to get air or you panick easily, then you need to get some more experience or some new snorkel gear.

For those of you ready for more advance diving here are a few tricks that will increase your underwater time.

1. Keep warm.  Body temperature plays a big part in the use of oxygen.  If you are cold or shivering you heart
must beat faster and use more oxygen to keep you warm.  If you want to impress someone - time
yourself underwater when you've been sitting in a hottub for a while.

2. Take a couple prep-dives when you get in the water.  Since you've probably been sitting in a vehicle your body
is only taking and using enough oxygen to get by on.  When you get in the water you will want to use and circulate as much air as you can.
Once you get in and are comfortable with your gear and setup, hold your breath for as long as you can.
 (Which will probably only be 20 or 30 seconds).  When you breath again you should be panting pretty heavily.
Float for a few minutes and rest for a bit while you catch your breath then repeat the process at least one more time.
For these prep dives and subsequent underwater dives you suffocate your system of air for a bit. 
It counters the problem by flooding itself with air when it becomes available.
You will feel your heart beat go up as your heart circulates the oxygen-rich blood around the body.

3. Breathe Deeply The passage from your lungs to the end of the snorkel is called dead air space.
Dead air is the air that you breath out - rich in carbon dioxide.  Each time you breathe in through the snorkel
you get all the dead air that you exhaled from your previous breath that was sitting in your throat, mouth, and snorkel. 
Usually when snorkeling, especially when nervous or uncomfortable it's natural to take short quick breaths.
When you do this you move the same air up and down in your throat and snorkel which results in a poor supply of oxygen.
If you can get in the habbit of breathing deeply you will get a fresh supply of oxygen for each breath and your lungs
will work at full capacity to get oxygen in the blood.  (Sometimes people try to breathe without the snorkel to minimize
the dead air space, but it takes more work to keep your mouth above the water and takes more oxygen anyway.  Just use the
snorkel - it will only help.

4. Relax  This step will come over time and experience.  Your ability to hold your breath is ultimately determined by you.  If you learn to control your body to relax, and your mind to think and to believe what you want it to, you will be greatly surprised of what you are capable of.  Try to relax your mind and enjoy yourself and your surroundings.  Don't paddle or kick at your max speed.  You will find that your can go further and longer while underwater by moving at a comfortable pace.

CAUTION  Skin diving and snorkeling are inherently dangerous.  Risks are mitigated, but NOT eliminated by training or skill. It is your responsibility to determine whether you have the skills and experience demanded by a particular dive location. If you are not willing to accept such responsibility then stay home.