You might as well start with your inbox: Click Inbox, switch to the Folder tab on the ribbon menu, and select Delete All. Once you've confirmed your choice, all messages in that folder will move to the Deleted Items folder. This category will automatically erase messages after 30 days, or you can empty it manually by opening it, heading to the Folder tab, and clicking Empty Folder. Repeat the process for your Archive folder, and then continue on with the others. To remove a category entirely, select it, go to the Foldertab, and choose Delete Folder.