*For mods that take place in the FreeSpace universe. If you want to play one of the total conversions that take place in the Babylon 5, Battlestar Galactica, or Wing Commander universes, you can skip this step!
Second, install Knossos. It will run on Windows, OSX, and Linux.
Third, browse Knossos for a mod that interests you, download it, and start playing!
For a more in-depth overview of FreeSpace, the Source Code Project, and the HLP Community, check out the Getting Started guide on the FreeSpace Wiki.