Don't worry about tags at the moment. Calibre does not download books for you. When you have an ebook file, for example a file with a .epub or .mobi extension, you import them by selecting them in that add books finder window.
You can import books you have purchased from Amazon, which will usually but not always have a .azw3 or .azw file extension. However, if the book is "protected" by digital rights management (drm), what you can do with it in Calibre is very restricted. You will not be able to read it in Calibre, convert it or edit it unless the drm is removed. We are not allowed to assist you with drm removal here at Mobileread, as it is against the law in some jurisdictions. We can, however, point you in the direction of Apprentice Alf's blog, which can help with drm removal.
https://apprenticealf.wordpress.com/