Civilization V Ultimate Modding Guide
Civilization V is very much flexible in terms of modding capabilities, there are already lot of Civilization V Mods released thanks to the massive modding tools released with the game.
Civilization series has always been mod friendly and the vary reason, it is supported by massive followers, which makes modding custom game types the core element to this platform. If you played Civilization IV, you will remember the famous Fall from Heaven series of Civilization IV mods, created by Kael.
You should thank this guy for releasing this information intensive Civilization V Modding Guide in PDF which guides you step by step through basics of modding itself to the real coding, with useful tips and tricks to walk you through making your first mode, with emphasis on it being fun and playable.
You can also go on to contribute and get help from a discussion thread Kael has started at CivFanatics. The idea of which is to get together, share your tricks, help each other and make incredible mods. What are you waiting for man ? go ahead and read if you are interested in modding Civilization V, consider it your first step to development.