The first thing you're gonna do is to have a plan how your planned civilization should look like. If you got the inspiration to create one, then the basics of modding begin
here.
You can find even more tutorials (such as creating an Icon for the Civ, or other things) within the Modding Tutorials and Reference sub-forum (where you posted the thread before).
Or refer to this
Modiki page for more details (you probably should refrain from coding Lua unless you're sure you can pull it from now on, but everything else should be writable in XML (or SQL, which is preferred since it copies from the general XML tables and thus may adapt to changes within the XML)).
EDIT: When planning a custom civilization, it's not necessary, but it might be a good idea to add Mod Support from other Mods.
JFD's Master Support File covers the general Mod Support Functions which you then adjust them to your liking.
If you have any other questions, feel free to ask me or one of the other Modders in the forum. I'm pretty sure they can help you with most problems.