Some smart TVs can have the NOW TV app installed on them, but not all.
If your TV uses the Android operating system (version 8.0 or above) and is made by (or branded as) Sony, Sharp, Philips, Panasonic, Bush, Hitachi, Toshiba, JVC, TCL, Technika, Cello, you can simply go to the Google Play Store to download the app.
If your TV was made by LG, from 2016 onwards, then it will have the NOW TV app pre-installed.
If your TV was made by Samsung, from 2016 onwards, and isn't a Samsung Serif model, you can download the NOW TV from its app store.
For all other TVs (both smart and non-smart) then you need to plug an external device into an HDMI port and access NOW TV via an app on the external device, rather than on the TV itself. (Such devices include those made by Roku and Amazon Fire TV sticks).
https://help.nowtv.com/article/get-now-on-my-device
To use NOW TV, you need to create an account online and then add credit (via a pass) to that account. Then go to the app and log in to that account to select and watch the content that you've paid for.