You should find a tutorial explaining about this, but heres the basic outline of what you need to do:
1. get the games files out of the mix files. These will be the .ini files. Youll need xcc mixer.
2. edit these files (thats, rules.ini and art.ini) with coding which is specific to what you want the tank to do and how it looks (as i said, find a tutorial for this, as its too hard and boring and really noobish not to look for a guide for it).
3. place them in your c:\westwood\RA2 folder.
The best way to get started and to get used to editing red alert 2 is to download tibed...
www.tibed.net
although when it comes to modding stuff and making mods, try not to go for using this.