You need to purchase the 10.6 (Snow Leopard) Operating Disks, as 10.6 is not downloadable. You can purchase these disks from the on-line apple store - http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard.
Make sure your computer can take the OS upgrade, by checking the specification required for the OS.
Snow Leopard System Requirements (10.6) - http://support.apple.com/kb/SP575.
Lion System System Requirements (10.7) - Mac OS X 10.7 Lion System Requirements
Mountain Lion System Requirements (10.8) - http://support.apple.com/kb/HT5444.
You also may want to Check your computers model and year, by entering your serial number in this link - https://selfsolve.apple.com/agreementWarrantyDynamic.do.