The upgrade from 10.5 to 10.6 (any version) is not free. You will have to purchase the retail disk set, which may still be available if you call the Apple Store. If they still have copies, the price should be just under $30 US. If they no longer have copies, you will have to look to private sellers who can charge whatever they think they can get. Once to any version of Mac OS X 10.6, you can then upgrade to the latest version, 10.6.8, at no additional charge.
You will have to have Mac OS X 10.6 before you can purchase OS X 10.7 (also now available only by calling the Apple Store) or 10.8.