what skills do you have? What would you like to do? For example, would you like to model? Code? Script? 2D Art? etc. Once you decide on that, you need to build up your experience.
If you would like to model, i would recommend getting Ms3d and play around with some vanilla models. Then you should get a trial for another program, 3DS max, Maya, etc and see which one fits your bill. You should practice modeling everyday. At first they may not look like you want them to, but read on some tutorials, google is your friend.
If you would like to do 2D art, i suggest getting Photoshop or Gimp. Read tutorials for them on the web. Try modifying vanilla textures
If you would like to code/script, i suggest you play around with the games text files. Make sure you back them up though. TWC has loads of tutorials for this section.
Once you have a good base in 1 or more of these areas, you should be ready to begin your own mod. I recommend that you take a few days to plan out your mod. Set goals for yourself. Have a well thought out, detailed plan. Once you are done with that, post a thread in the proposals forum. You may also want to have some work done, if you can.
I hope this helps
