How do i change my password?

First, click on "My Card: " (all the way at the top and near the right edge of any page). Then click on the Options tab and 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 Files (and images) by creating a card and setting its card type to "file" (or "image"). Then you will see a standard interface for uploading. You can then include files or images on other cards.