Hello Ina,
in addition to Ned's hint:
First of all you could have a look there: https://www.adobe.com/account.html >>> see My productsView all >>> These registered products are associated with your Adobe ID. >>> Name >>> Serial number
If not successful use (Ned's link) http://helpx.adobe.com/x-productkb/global/find-serial-number.html >>> I lost my serial number >> see comment
And because i'm not sure: Maybe you obtained your "serial number" from these firms like Amazon. This is not a serial number, only a code with which you can request a serial number from Adobe. Here you have to go through your "Adobe Store", and look for the button "Serial Number Request". Fill in the form and, after a while you will get a "real" serial number.
Good luck!
Hans-Günter