If you are not going to continue with the one month sports pass after the £12.99 offer, then make sure you cancel before the next due payment date otherwise you will be paying £33.99 each month until you cancel.
Would suggest cancelling a couple of days before the expiry date of the offer and don't worry if you cancel early where you will still get the full one month duration on the sports pass,
If you do cancel, make sure you follow all the step by step on screen instructions until you get the thumbs up cancel confirmation on screen and hopefully an email from nowtv confirming you have cancelled your pass.
What's the make and model of your smart TV ?
If you don't own a device on this help guide, then either a nowtv black box or nowtv smart stick would be the cheapest option from Curry's or Argos assuming your TV has a spare HDMI port free.