How do i make a value go up by 1 when i kill someone?

When players join i insert a Number value into their player. And When they kill someone i want the value to go up by 1 in their player. The value is called exp. How would i make it so that it goes up by 1 every time a player kills another player? Dont worry i already have script that inserts the value into the player that was added to the server.

Look at leader board scripts thats all I can Give advice for, Since you did not provide a script.