Although there is so many of these tutorials on youtube, I will post a short and sweet version here.
First get a timer. Then a button. Wire start of the timer to the button to the timer. Then get a floor round down gate. Wire the floor round gate to the timer. Wire an equal gate to the floor gate and to a constant value of say 5. Then wire an explosive to the if gate. Then when you press the button, the the timer will start, and when it equals 5 then the explosive will blow up. If you want then you can wire the floor gate to a screen. You may also need a button for reseting the timer.