To start helping the science community with your very own computer, follow these easy steps.
1) Download BOINC. Works on Mac, Windows, and Linux. http://boinc.berkeley.edu/download.php
2) Attach to a project. Add project URL to BOINC in the Tools menu. Tools -> Add project... http://boinc.berkeley.edu/projects.php
3) Set up an account with that project, and away you go.
You can adjust settings within the BOINC client if needed.