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.
Not sure if this is your problem? See the articles 'Troubleshooting Gas Hot Water Heaters' and 'Electric Water Heater Troubleshooting' for a listing of all of the water heater problems and solution resources.
Just because you see water droplets on the pipes or the top of the water heater does not necessarily mean you have a leak. It could be condensation. Cold water lines will sweat during hot humid conditions. At times this water may run down the pipe and puddle up on the top of the tank.
Before you assume that you have a leak, dry off the pipes and try to determine where the water is coming from. If the only water is coming from condensation on the pipe, then you don't have a leak.
For gas hot water heaters there may be another type of condensation if your vent is not operating correctly. If you are getting a back draft from your vent, it can cause some condenstion and staining on the top of the tank. See the article on 'Venting Gas Water Heaters' for instructions on what to do.
Types of Leaks
When we are talking about the piping for water heaters we are generally talking about water lines. Although on a gas water heater you will also have a gas line. Gas lines have fittings to and can leak, but that is a different subject.
The only other pipe on a water heater that can leak is the drain valve. For a discussion on how to stop the drain valve from leaking, see the article, 'Repairing Water Heater Drain Valves'.
Water Pipe Fittings
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.
Water Line Valves
You should have a valve on the cold water inlet side of the tank. Valves have washers in them and can start to leak around the valve stem. See the article 'Fixing Leaking Valves' for instructions on how to fix this problem.
If you do have a leak, tightening the fittings should solve the problem. It may be that you have some condenstaion, lowering the humidity in the area where the water heater is will solve this problem.