Have you gone through the FAQ's in the Sony Help section? I opened the Reader for PC software on my computer & clicked on Help. It took me to this page which I think would be useful for you to go through.
http://ebooks.custhelp.com/
Look at the questions pertaining to creating an account, registering the device, Adobe ID, authorizing, deauthorizing & things like that.
**edit**
Please note that some of the terminology is for the previous Sony Reader software, Reader Library. Terminology for the Reader for PC/Mac will be similar but not exact.