How do I find the serial number online?
Go to Apple - Support - Welcome To My Support Profile ... assuming you registered your Mac.
Also, is there a way to activate Find My Mac?
"Activate"? Are you literally asking if you can set up Find My Mac after the theft? No! But if you mean you already set it up and want to use it, then here's how to do that:
Go to http://www.icloud.com/find on any computer's web browser. Sign in, and find your Mac there and do something (like erase it.