Provided that there aren't other issues with your heater, changing the t&p may be enough. I say this as they generally do not just fail on their own, but rather, something has caused it to operate. This is either caused by temperature or pressure. Without knowing the age of your heater, it is a bit difficult to advise you as to replace or repair the heater. If you are comfortable that your's has more useful life in it, you can replace the t&p valve. If the expected life of the heater has been reached, you may wish to replace the unit.
To Replace the T&P Valve: Shut off the gas, and close the water inlet valve. Drain the tank down. The best way is to attach a hose to the boiler drain and route it to the outside. This also will help remove silt and sludge that can build up in water heaters, and extend the life of your tank. You will need to take an air bleed to allow the tank to drain. You can do this by lifting the lever on the t&p valve and by opening one of the hot water valves that you heater provides water to. You should hear air being pulled into the heater as the water drains.
There is a tag on your valve. When you remove your t&p valve, take it to the hardware store with you, tag and all. Get an exact replacement, and install according to the directions.
Before you pressure the heater back up, flush water through it through the still open drain. This will stir up and flush out any remaining sediment.
Close the drain, but leave the hot water valve that you opened earlier open, as the tank fills, the air will be expelled down the line. Fill the heater completely, then re-light the heater.
Best regards, --W/D--. Feel free to rate this solution.