Unless things have changed in the past few years, mods for sections are chosen by higher staff. People used to say "Don't ask to become one!" but I don't see that as a problem because it shows some interest in PC.
One of the best ways to help in the section in to post in threads. Answer people's questions correctly, and make sure to give them good advice. Keep a good attitude while doing it too, since some times, you'll see the same questions pop up all the time.
If a post breaks the rules, report it! Don't deal with it yourself, because you don't have the powers for it. With reports, that helps icycatelf keep the section running well.
The one thing I should warn you about is that mods are added as needed, especially if the section is popular. Unfortunately, since the forums are slow, higher staff might think that the TCG section is doing well with just one mod for now. Please do not let this discourage your enjoyment of posting in the TCG section! It's just a fact of the way things are now on forums (but things can always change).