Well every water heater of a different company has its own way of turning on heater manually. My water heater has a pilot knob, I press it and light the pilot, while I keep pressing it, I turn on the burner through the thermostat dial. For more specific details please mention your model.
There are two of way to lite a H/w/h . If it's not electrician pilot liter . Then take a fire place matchs or along pc. Of paper hold pilot knob for ten sec. & then turn knob over to lite . Then you should be o.k. if its a elec lite it may take a couple of times pressing the switch . take off the plate in front of the H/w/H. At the bottom look in side way in the back you will see a funny looking small pipe inside that is the pilot tube. Place the match to that & this should work .