The key lies in an understanding of the operation of a PLC.
First, a PLC continuously executes its program, evaluating each rung of ladder logic in sequence.
Second, it updates the state of the input signals before executing the program.
You need two rungs. The first rung uses the signal from the pushbutton to turn on an output, but only if the signal from the pushbutton was off during the last scan. The second rung is used to remember the state of the signal from the pushbutton for use during the next execution of the program.
See if you can convert that verbal description into ladder logic.