Here's a few suggestions:

As an introduction to text files working, try ADDING a new unit.

Download the bonus Spartan model from the official RTW website, and make it work in game, replacing the stock Spartan Hoplite.
This is an easy task, but it will show you the relationship between the physical 3D model and the text/stats

Next....make it a stand-alone unit so you have BOTH Hoplite's in game.

This will require you to copy chunks of data from the key unit text-files, and re-name the bits that make a unit unique.

When you have managed to do those things, you will know which text files make up a unit, and how it all fits together. After that, you will want to work out how to build that unit in a campaign by editing descr_building.

At each step BACKUP ANY FILE YOU WANT TO CHANGE

Also, add -show_err to your startup shortcut. If you screw up a text file, this will tell you which one and where!

After that...you could experiment with removing units.

I recommend working on ADDING a unit before you remove one. It's a lot easier to tell if you have done something correctly that way.

Tutorials to do ALL of these things exist in the Scriptorium, or over at twcenter.net in their tutorial section.

Happy Modding