If you want to add new techs, units, buildings, etc., then you need to create a content mod (mostly XML, but you'll need some .DDS files for the icons and some modifications to the art defines files for new units). It's a LOT of work to get it right, although you can probably hack together something workable without going too in-depth on the graphical bits.
Go to the Reference forum and read Kael's modding guide. It's fairly complete, with the exception that it predates the VFS change (which you'd need to use for the DDS files and art defines). That should explain the basics of XML and SQL modding, and with those you should be able to get started on adding custom techs, units, and buildings.