Getting Started for Dummies

All of my ebooks (1500+) are Kindle and I read them on an Android tablet which is designated as my default device.

Can someone please just tell me how I get all those books into a calibre catalog? The User's Guide simply says "Drag and drop a few e-book files into calibre, or click the “Add books” button and browse for the e-books you want to work with." Drag and drop from where? Add from where? What am I missing?

I'm really not a stupid person, but I'm starting to feel that way.

Thanks so much.

skp