Go on the developerworks site:-
You will have to create an IBM ID and password as well as a profile, then you can download the fulll Client / Designer / Admin set as well as the server software.
IBM will send ytou links to a whole series of resources and papers to help you understadn how it all goes together.
Another good spot is the main Notes.Net home page with access to other downloads, forums info etc.etc.
Hope that this helps.