Do I turn off the water and the gas if my hot water tanks starts to leak. How/ where do I turn off the gas if I need to do that.

1. Turn off gas. Every hot water heater has small gas valve on its left side and about 16" up from the floor - usually, but not always, connected with the heater via Flexible Gas connector line.
2. Close cold water. There should be water valve on the right side of the heater.
3. Release pressure. Once closed, attach hose to the valve located on the bottom of the heater and open it. Drain the heater.
4. Replace heater. Best is to replace old heater with approximately the same unit. That way installation requires least work and plumbing modification. Open cold water valve. Fill heater with water.
5. Follow the instructions pasted on the front of the heater to start it up.

Milo has given you good instructions how to change the water heater.

I will add the link below to show you where the leaks come from, the image on right show a gas water heater and you will see the gas line connected to the S/O valve and the gas valve, but most S/O valve are connected to gas valve via Flexible Gas connector line, good luck.

John

Home Tips : Water Heater Leaks