You can set up parental controls or a pin by visiting the system settings options on your TV.
How to set up parental controls or a PIN
This varies according to your TV but can usually be found under the settings > system options on most devices.
For full instructions on how to set up parental controls or a PIN, it's best to check your TV user manual.
If you can't find it, search for your TV model number on the manufacturer's website. Most have manuals that you can download or read online. If yours doesn't, get in touch with your manufacturer who may be able to send you a copy.