Before you can log in to Plesk, you will need to navigate your browser to the URL that should have been given to you by your hosting provider.
Enter your username and password. (If you'd like to change the interface language, use the drop-down menu.)
Select "Log In".
You are now logged in to your account and can mange all the various aspects of your hosting subscription(s).
When finished using the hosting panel, be sure to log out to prevent unauthorized users from gaining access by clicking "Log out" at the top right of the panel.
You now know how to log in and out of Plesk.
For an overview of the most important features in Plesk, check out the following tutorial, "Becoming familiar with the Plesk interface."