You would want to start making decks or art yourself, create a portfolio and show how your designs are better than what is out there already.
Theory 11 deals with high profile customers. Jimmy Fallon, SNL, NPH, J.J. Abrams etc. These are A-listers. They employ world renowned designers, and A-list artists to create their decks.
If you want to create a deck design for Theory 11 you need to establish yourself as an incredible artist. There are other sites out there that might take user submitted designs. Art of magic, or House of Playing Cards might. If you want to design playing cards for a living though you don't have to do it on Theory11. You can start right now, the USPC makes it fairly easy to design your own cards. You can sell them online or at live events. This is all really difficult but it might be the most direct path to doing this.
If you just want to do a lot in the magic industry learn a lot and become great at one skill. Magic websites need photographers, web designers, accountants, video editors, computer programmers, advertisers, marketing people, magic creators, and performers. Focus on one of those areas, like Christopher already said, if they have an opening they'll post it or contact you directly.
Jonathan Bayme wasn't your average 16 year old. If you are average be honest with yourself and look at how you can become exceptional. Plan and create something that will prove that you are better than everyone out there.