There's no need to sign out of your account when you've finished your work, if you're using a secure computer. However, if you're on a shared machine, you should sign out when you're done, to maintain the security of your organisation's information.
To do this, open the bottom menu from the navigation sidebar on the left-hand side of the window.
Select the "Sign out as" option about half-way down the menu and you'll be logged out. You will be taken to the home page, and you will see a green notification indicating that you've been successfully signed out of the system.