From the API FAQ's: https://www.themoviedb.org/faq/api

How do I apply for an API key?

You can apply for an API key by clicking the "API" link from the left hand sidebar within your account page. You need to have a legitimate business name, address, phone number and description to apply for an API key.

In other words, create an account on themoviedb.org, on the account page, click the API link on the left side and follow the instructions from there.

Thank you for you reply :)