A leaking water heater drain valve may be easy or not so easy to fix. The age and quality of your tank are both factors.
You have a three options with the leaking valve, one would be to try and tighten the fittings on the valve, see 'Tightenting a Water Heater Drain Valve'. Second would be to put a cap on the spigot, this only works if the leak is small. See 'Capping a Water Heater Drain Valve'. Third, you can replace the valve, this is a bigger job. See 'Replacing a Water Heater Drain Valve' for more information.
For a list of all the water heater topics, see 'Troubleshooting Gas Water Heaters' and 'Electric Water Heater Troubleshooting'.
Near the bottom of your hot water tank you will find the drain valve. From the factory your hot water tank comes with a plastic valve. These valves are not high quality and sometimes they start leaking when you use them. How serious this is depends on how bad it is leaking and where it is leaking.
The first thing to do is open the valve a little and then shut it back off and tighten it. This will release any sediment that may have caught in it. Did this fix your leaking hot water heater? Yes, great, you're done.
Where is the water coming from. Is it coming from the spigot? Is it minimal? You could take the easy way out and put a cap on it. A couple of bucks at the hardware store.
Is the spigot leaking a lot? Or is it leaking where it connects to the tank? Yes to either of these means you should replace the valve. Go to the article 'Replacing a Water Heater Drain Valve' for information on changing the valve.
WARNING!! Be CAREFUL the Water in Your Hot Water Tank is VERY HOT!!! WARNING!!!