Thunderbird does not use the word use the word spam in a effort to differentiate itself from other software that marks items as such. As you have found it uses the word junk in an effort to show what it has decided is junk mail. Unfortunately some providers, like outlook.com, has started using the word junk and junk folders to confuse things like Microsoft does best. You can add a Junk button to the Mail toolbar by right clicking the Mail Toolbar and selecting Customize. Find the Junk button in the dialog box that opens and drag it to the toolbar and drop it there. You can use the shortcut of j to mark as Junk and shift+j to mark as Not junk. You can click in the Junk column on the main screen to mark and unmark junk. The Junk column has a flame in it's header. You can use the Message menu and select Mark As Junk. As you can see there are several ways to get there. The default action for Thunderbird is to mark items as junk with the icon in the junk column and leave it in the inbox. You can got into Junk Controls to change that behavior. As you are new to Thunderbird remember Right Click is your Friend. Many options can be accessed using the Right Click.