Just from personal curiosity, which web pages did you visit with your question that had you end up here in the 10.3 and earlier forum Circa 2003 obsoleteware?
It seems a common fault with this Apple Spoort site that so many posts are made to this forum that are irrelevant.
Can you post a few links from your browser History, just for research?
To answer your question, to upgrade from 10.5 to 10.6 you need to purchase the 10.6 retail DVD such as from here.
(For your home country use the Apple localised site.)
The DVD is reasonablly priced, and once installed you can upgrade your iTunes etc.