presuming that its an old style vaillant with a permanant pilot flame then there should be a largish round grey knob with a picture of a flame on it, push this in and keep it pushed in, this releases the gas to the pilot head, then if the boilers electronic you will hear the spark electrode clicking, if it isn't electronic then you will need to rapidly depress the ignition knob which is usually red or orange until the pilot lights, in both cases once the pilots lit you keep holding down the grey round knob for about 10 to 15 secs and then slowly release it, the flame should stay lit. if it doesn't wait 3 mins then try again but holiding the grey knob in for longer, if it still doesn't stay alight its knackered!