Tightening a water heater drain valve does have a chance of succeeding. Unfortunately, on some water heaters the valves are not good quality. Attempts to tighten them may result in damage to the valve.
The other two options for a water heater drain valve are to replace it, or possible cap it. See the articles 'Replacing a Water Heater Drain Valve' and 'Capping a Water Heater Drain Valve' for information and instructions. Not sure if this is your problem? See the articles 'Troubleshooting Gas Water Heaters' and 'Electric Water Heater Troubleshooting' for a complete list of all the water heater topics.
Shutting Down the Water Heater
Shut the power off or turn the gas valve to Pilot. Run enough hot water in one of the sinks to cool the water down some. The water in your tank can be very hot. Be careful, see the articles, 'Draining a Hot Water Heater', 'Shutting Down an Electric Water Heater' and 'Gas Water Heater Shut Down Procedures' for more information.
WARNING!! The water in your hot water tank can be VERY HOT. Make sure the water is cool before you open any valves. BE CAREFUL!!! WARNING!!!
Tightening the Valve Handle
The first thing you want to do is to try and tighten the valve. Get a small pail to hold under the drain valve. The water heater drain valve has a rubber washer in it, similar to many valves in your home. The idea of tightening it is to get it to seat better and stop the leak.
Get a screwdriver and open the valve. Keep the pail under the spigot. This will blow any sediment out of the seal. Allow enough water to spray out to give it a good flushing. Now tighten the valve.
Did the leak stop? Yes, great. No, try it again and see if it seats the second time.
Tightening the Valve Stem Nut
Another spot that the drain valve could be leaking is at the valve stem. If you see water coming out around the stem, you can tighten the nut. You will need an adjustable wrench.
Carefully tighten the nut, you should not need to turn it very much. Check to see if the leak stopped.
Still no good? How much is it leaking? Just a few drips. You can go to article 'Capping a Water Heater Drain Valve' and see if you want to put a cap on it.