This Should ONLY be preformed by a licensed professional
The worst scenario you can imagine is smelling gas in your home. You go from room to room to try to determine where the smell is coming from. Any gas leak is potentially dangerous. Chances are the leak may be coming from your old gas fired water heater. Here are some steps to show you what the licensed plumber will do when he arrives to fix your leaking water heater. The Plumber will Go to the Hot Water Tank and take a good sniff in. The Plumber will use a spray bottle and fill it with soapy water. It sounds strange but this method actually works. Spray around each pipe joint. If you have iron pipes you will see tiny bubbles form. If you have Copper pipes you will have to find a kink in the pipe and spay the pipe. The Plumber will Look closely and see if there are any bubbles at any place on the kinks or the joints of the piping. You will need 2 wrenches to fix the leak.
The Plumber will Take one wrench and place it on the side of the pipe and then on the other side of the pipe place the second wrench. Tightly secure the pipes. Make sure only one pipe moves when you are tightening. The Plumber will Re-check for leaks by spraying the soap again. If there are no bubbles then you have successfully fixed your leaks. The Plumber will have tightened the pipes, you may want to use some joint compound or some Teflon tape to secure the pipes. If this method did not fix your leak, you may have more than one leak. Turn off the gas and add more joint compound and retest.
These are quick fixes to your leaky water heater but you may be dealing with a more serious problem if you have an old water heater. You should have your boilers and water heaters tested before heating season. This way you don't encounter any problems. The Plumber may want to replace the old water heater if it is over 10 years old or if you spending more money to run the water heater than you are on your heating bills.