How do I change my password?

First, click on your name (at the top, near the right edge of any page). Then open the menus - the gear icon at the right edge), and click on the last item, "account". You'll see two text boxes labeled "Password" and "Confirm Password". Just fill them in with your new password, then click on "Save Changes".

Can I upload files? Are they linked to a card?

Yes, you upload File (and Image) by creating a card and setting its types to "file" (or "image"). Then you will see a standard interface for uploading. You can then include files or images on other cards.