How do I get a network security key for my Internet

The password/key for a Wi-Fi network is controlled by a setting in the base station (router). So, if the network belongs to someone else, ask the owner/operator for a password.

If it is your own Wi-Fi network, and you have forgotten the password, it may/will become necessary to gain access to the base station's user interface. The way this is done depends on the make and model of the base station. For Apple devices, a special application is used, whereas many third-party routers allow access via a standard web browser.

It may help if you could provide information about the exact base station model and about the type of computer (and operating system version) used.

For Apple devices, you may want to have a look at the following user tip by Tesserax to begin with.

AirPort - Regain Access from Forgotten Passwords

See also the following article (which could be useful, for example, in a case where one may have to carry out a factory default reset).

Resetting an AirPort base station FAQ - Apple Support

I need the network security key for my Internet to connect. How do I find that

We cannot tell from the information in your post whether you are trying to connect the AirPort Express to a wireless network provided by another router.......or......whether the AirPort Express is in fact providing a wireless network at this time....and you are trying to get another device to connect to the signal from the AirPort Express.

Can you clarify on that please?