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.
Also, make sure that your computer can in fact be upgraded to the newer OS version. Check your computers model and year - https://selfsolve.apple.com/agreementWarrantyDynamic.do.
Once you know the model and year and you are considering upgrading to the current OS (10.8) ensure that your model is supported for the 10.8 OS - http://www.apple.com/osx/specs/.
Also if considering 10.8 - remember to ensure that all your other applications are supported by the new OS - http://www.roaringapps.com.
Snow Leopard System Requirements - http://support.apple.com/kb/SP575.
Lion System System Requirements - Mac OS X 10.7 Lion System Requirements
Mountain Lion System Requirements - http://support.apple.com/kb/HT5444.