To use this service, you must first register. If you don't have login credentials yet, you can of course register. The registration itself can be completed in just a few minutes. You then need to request and store an API key from openweathermap.org.
To test the service, you can also log in with the username 'demo' and the password 'demo'. Only a few locations are available for selection, and the units are fixed.
You can register here.