The Files page can be used to upload or manage documents and is accessed via the Setup menu at the top of your screen.
Attachments can also be uploaded and added directly to an entry via the Information section:
From the Files page, simply click on the add new attachment button to select a file to upload. Documents are restricted to 15Mb per upload, which is enough to handle most file types.
If you want to edit the title of a file just click on the text and type your changes, or if you wish to remove a file, simply click on the delete link next to the file name and confirm when your browser asks you to.