A leaking hot water heater can result from leaks in the pipes. At the top of your hot water heater you have an inlet and an outlet pipe. The cold water comes in and hot water goes out. There is usually an union on each one that allows for the removal of the hot water tank.
For leaks that occur at the unions you will need to tighten them. Use two pipe wrenches to do this. You put the pipe wrenches on in opposing directions. It will take a little muscle to tighten them.
The other place that the pipes might leak is at the nipple where the pipes enter the tank. Are the connections old and corroded? This could be a problem. The tank may have started to rust. Messing with this connection could render the tank unusable. Their is not much you can do if you damage the threads going into the tank. Do they seem OK, just leaking a little?
You can try tightening them. Shut the water off on the cold water inlet. Open a valve in a sink near by to drain down the water in the pipes. You will need to loosen the unions above the nipples. With the union loose you can tighten the nipple with the pipe wrench. It will take some force, but be careful.
Don't go much more than a quarter to a half a turn. Still leaking? Nuts! Go the other way and take the nipple out. Clean up the threads and put new pipe dope or Teflon tape on them. Put the nipple back in and tighten it up. Tighten the union and turn the water back on. This should take care of a leaking hot water heater, when the nipple is leaking.