Capping a water heater drain valve is sort of like kicking a can down the road. It isn't really fixing the problem. It will stop the drain valve from dripping on the floor temporarily. Eventually, you will have to fix the valve.
Would you rather get it fixed now? See the articles, 'Tightening a Water Heater Drain Valve' and 'Replacing a Water Heater Drain Valve' for instructions on how to repair the problem. Not sure if this is your problem? See the articles, 'Troubleshooting an Electric Water Heater' and 'Solving Gas Water Heater Problems' for a complete listing of all of the water heater repairt topics.
Capping a water heater drain valve will only work for minor leaks. If the valve is leaking badly, you will need to repair it. This also only works if the valve is leaking through the spigot portion. If the leak is around the stem or fitting going into the tank, this will procedure will not work.
Purchasing a Cap
The cap that you would use it the same type and size as a cap for a garden hose. Any home supply store or hardware store will have one. They only cost a dollar or two. The caps can either be made of metal or plastic.
The cap has a washer in it just like a garden hose. When you tighten down the cap it will seal the end of the drain valve and the leak will stop.
Installing the Cap
Put the cap on and tighten it with the adjustable pliers. Not too tight, remember it is just plastic. That should do it, you have stopped a leak on your water heater drain valve.