You already have a topic about this. Also, please don't PM me for help, you could've just asked for more details in the topic.
Rules.ini is a text file which contains information about the game's settings, units, animations etc. It is used by Red Alert 1, Tiberian Sun and Red Alert 2.
Go to
http://xhp.xwis.net/. Click "Utilities" from the left sidebar. Then choose XCC Utilities. Download and install it.
Then launch XCC Mixer (You can find it from the start menu) and read
this guide about using it (Yeah, that might not be the best guide around, however that's how I learnt to use XCC Mixer).
To get rules.ini, click File -> Open in XCC Mixer, browse to your Tiberian Sun directory and open patch.mix (assuming you have the latest version of TS, if patch.mix doesn't exist then open TIBSUN.mix instead, which is also located in the TS folder (like all of TS's mix files)).
If you opened patch.mix, you should see the rules.ini in the left bar. Right-click on it and select "Extract". Browse to your TS folder and save it there.
If you opened TIBSUN.mix, you should see lots of mix files in the left bar. Find "local.mix" from there and open it (by double-clicking it). Then you'll see hundreds of different files. Find rules.ini (the files are ordered by the type, and rules.ini's type is "text"). Right-click on rules.ini and select Extract. Save rules.ini to your TS directory.
Then, like I said in the previous topic:
QUOTE
2. Open rules.ini with some text editing program (Notepad) and search for "Mammoth Tank", or "4TNK" (search by pressing Ctrl+F)
3. Find the "TechLevel" key below the "4TNK" unit. Edit the "TechLevel" value to something between 1 and 10. Save the file, overwriting the previous rules.ini.
4. optional: by default the mammoth tank isn't very useful compared to it's cost and other things (it also requires a tech center and is slow), so you might also want to change some other things like Cost, Sight, Speed etc.