How to reset my Mac back to factory settings ?

Like this:

http://support.apple.com/kb/HT5943

Barry

What to do before selling or giving away your Mac. This link provides information for systems that came pre-installed with OS X Lion, Mountain Lion and Mavericks.

As your early-2011 MBP came pre-installed with OS X Snow Leopard, you will want to boot from your SL install DVD; then use Disk Utility to format your internal HDD and finally, reinstall OS X Snow Leopard. After the install completes, you can turn off your MBP.