There are detailed instructions on the water heater, text and diagrams, but the following is my recollections.
If the gas tap at the end of the house supply piping is turned off, the knobs on the heater will be in the operating positions.
Turn the main control on the tank off and then turn it to a position that will be labeled "Pilot". (Maybe just turn it to "Pilot".) Open the cover on the burner compartment so you can see inside.
Turn the smaller "pilot" valve to a position (usu 90 deg) which will allow the valve to be pushed down against a spring. Holding the valve down opens a safety valve and allows gas to flow to the pilot only.
While holding this valve down try to ignite the pilot by pressing an igniter button. Observe whether the pilot lights. If it does, continue to hold the button down for 15 seconds or so after the pilot lights so that the heat of the pilot can warm a sensor that allows it to stay on. If you release the button too soon, the pilot will go out.
If the pilot does not light with that press of the igniter, then continue to hold the pilot valve down, wait 15 sec, and press the igniter button again.
Turn the pilot valve to the operating position.
Turn the main valve to its operating position and desired temperature. When you do this the main gas burner should come on and start heating the water in the tank. In 30 min or so you will have usable warmed water in the tank.
Close the cover on the burner compartment.