Replacing a water heater pressure relief valve is not a difficult repair. It will require that you shut down the hot water tank prior to starting the repair.
Are you sure that you need a new pressure relief valve? See the article 'Checking a Water Heater Pressure Relief Valve' to see if yours is bad. Not sure if this is your problem, see the articles 'How To Troubleshoot a Gas Water Heater' and 'Electric Water Heater Troubleshooting' for a listing of all of the water heater repair problems.
Purchasing a Pressure Relief Valve
Before replacing a water heater pressure relief valve, you need to get a new one. They are less than $20 at a home or plumbing supply. There are different pressure and temperature ratings. A standard one would be 150 psi and 210 degrees for temperature. There should be a little plate on the valve that gives you these figures. Take the old one with you and get one that matches.
Shutting Down the Water Heater
To change you water heater pressure relief valve you are going to need to drain the tank down part way. Turn the breaker off or turn the gas valve to pilot. Run some hot water in one of the faucets until the water is lukewarm.The water coming out of the tank will be pretty hot if you don't cool it off. Shut off the cold water inlet valve. Hook up a garden hose to the drain valve at the bottom of the tank. Run the hose to a floor drain or bathtub nearby. You need to drain the water down below the water heater pressure relief valve.
See the articles 'Shutting Down an Electric Water Heater' and 'Gas Water Heater Shut Down' for instructions on the correct procedure to follow. You can damage the internal parts on a water heater if you do not shut them off properly.
There is no good way to tell what the water level is in the tank. Go with how long it takes to fill a bathtub. That will be plenty. If water starts gushing out when you loosen the valve, you need to drain some more water.
Installing the New Relief Valve
You will need a pipe wrench to remove the old water heater pressure relief valve. You may or may not have a outlet tube attached to it. Remove it first. Save the tube you can re-use it with the new valve. Clean it up, as required.
With the outlet tube removed you can take out the old pressure relief valve. It will take a bit of effort to get it loosened. Remove the old valve and clean up any residue around the opening. I am not aware of any arts or crafts that use old pressure relief valves, so you will probably just want to throw it away.
Put some pipe dope or Teflon tape on the threads for the new water heater pressure relief valve. Thread the new valve into the opening and tighten it down. The outlet should be turned down. Put a little pipe dope or Teflon tape on the nipple for the outlet tube and then install it.
Starting Up the Water Heater
Similar to the procedure needed to shut down the water heater, you also want to follow a procedure for properly starting your water heater. See the articles 'Starting Up an Electric Water Heater' and 'Gas Water Heater Start Up' for instructions on the correct steps.
The usual solution is to replace the valve. It is not that big of a job and the part is not that expensive. You should have paid about $15 dollars for a new valve. Hopefully you were able to deal with your valve.