Hello and Welcome
Well to save me typing everything out, I would point you towards a good guide (with links to other useful guides at the bottom)
RA2FAQ » Kravvitz's RA2/YR Modding Tips and Answers to FAQ's
To get to the INI files you need XCC Mixer by Olaf, that can be got from here:
XCC Home Page by Olaf van der Spek
Once you've got this you can extract the .mix files found in the game directory and this will give you access to all the files you'll need to edit the game.
If you have any further questions after reading through the guides feel free to ask.