The google word is 'monostable' and most links will cover the 555. This one
was the first good one listed and is the simplest circuit. A minor snag is that if you keep the button pressed the timer stays on.
This one adds an extra couple of bits to give a fixed on time even if you sit on the button and shows how to add a transistor switch to boost the current rating. On Time = 1.1 x R1 x C1 (1.1 secs with 1M & 1uF)