how do I upgrade form ox 4.11 to leopard

My system has not automatically updated itself and when I check for updates it says that there are none; however, I can't do anything because it is requesting that I have OX 10.5 or higher. I currently have 10.4.11 and have no idea how to update it. I tried updating to 5.1 but it says I need 5.0. I can't find 5.0 except the Leopard update and the download is not there. I need help!