Hi there Calo,
The first step is try some simple modification by yourself. You may already have done this, but if you haven't:
Start small. Try changing a characters name, try changing the stats of a unit.
Build from there. Try adding your own new unit using exisiting models and textures, try editing the campaign map slightly.
You can see where this is going. Start small and work your way up. There are loads of tutorials out there to help. If you need help with something specific, myself or I'm sure someone else here can point you to a detailed and good tutorial.
Once you've done some modding yourself, mod teams will be willing to have you, and that's when you'll get real experience of modding the game. Good luck