Welcome here Vojvoda Here are some threads that you should read before asking for help:
ADDING UNITs TO C3C: Thorough Guide for the Newbie - with this one you can already do a lot of work by yourself. Shorter version (a bit different too): How to add unit in C3C epic game. Mini HOWTO.
Then there are another lot of things to do but you need an image editor for that part. For example:
How to add resources to C3C - A guide
Tutorial: Making a units_32 pic
I would also recommend you to choose another editor among the following ones:
Steph's Expanded Editor for Civilisation III
Cross-Platform Editor by Quintillus
Hope that helps Believe me, you can do a LOT of things once you have the right tools and understand how modding works.