Water heater pilot light goes off after heating one tank of water and has to be relite again.

That's usually the thermocouple and you will need to get a universal thermocouple kit at any good hardware store like Home Depot or Lowe's. Make sure when you replace it to adjust flame to hit 3/8" on tip of thermocouple only. Any more than that and it will not work. It works on the temp difference between tip and rest of thermocouple. The only other thing that will cause this problem is the gas valve. If you have a good ohm meter check between inside center of where you screw in thermocouple to body of gas valve. You should get a high reading if it's good and infinity if it's bad.