Gas hot water tank leaking - water turned off, what about gas? Repair or replace?
The straightforward solution is to call a plumber to replace the heater. If you are not skilled at sweating copper and have the equipment, then this job is too difficult. 

If I were going to repair the leak without turning off all the water to the house, I would cut out enough of the tubing on the tank side of the valve to allow unscrewing the copper female adapter. Then clean up the threads and solder up a new fitting with tubing so that you are not heating the water heater tubing. Then screw it on and finish by soldering the other end.   

Before cutting into the tubing, you would turn the gas at the heater to pilot, then turn the cold supply off. It appears the leak is on the cold supply side. Open a hot tap somewhere to relieve pressure in the tank and drain the tank down a little.