how do i create a mod

Well, that's a pretty broad question. I'm pretty new to this game but here's what I know so far: There are several different things you can change/create. As for new additions to the existing game, you can create new Events, Anomolies, United Planet Issues, Polotical Parties, Technoligies, Ships, Starbase Modules, Buildings, Resources and so on. You can also modify the graphics, but it takes a LOT more work, and you'll need some tools.

You can visit NeoTech gaming's download site and, Under Galactic Civilizations, get a few tools:
http://www.neotechgaming.com/forums/index.php?act=dscriptca

Graphics Editing Suite is a must. It'd also help a lot if you had a graphics program that supported full-color PNGs with alpha channels, instead of lousy 256 color PNGs with background layers like my hokey version of Paint Shop Pro... Supposedly GIMP does this, but I haven't installed it because it doesn't like my computer.

If you can get GalCivWriter working, that'll make it easier for you to modify/create text-based events and such.

For Political Parties and Starbase Modules you need to take care of a propper numbering. This was the source of a lot of pain.

In general you need to be carefull with text placeholders. When you change the number or the order of them it can lead to a CTD (crash to desktop).