If you are on a SIM only deal, just ring 789 and request it. Your service will be terminated at the next refresh date and the last payment will be taken from your bank account about two weeks later.
If you have any minimum term left to "serve" - the last payment will include the monthly amount from the time you cancel to the end of your minimum term.
If you took out your contract within the last two weeks, you're still in the "cooling off" period and can cancel without penalty (you'll still need to pay for any calls or texts you made outside of plan).
If you want to keep your number, you should remember to request your PAC when you cancel.