All those things that you have posted are careers my friend. Game development is the field in which you would carry out such careers. Assuming it is gaming you want to do of course. So if you want to be a character modeler, then start learning to model characters. Get a good grip on human form and proportions and anatomy. If you want to be a level designer, then work on your understanding of perspective and create yourself a mod. If you want to be a texture artist, take lots of different pictures of textures around where you live or where ever you may venture off too. Get really familiar with photoshop. THere are many different ways to study all of these different careers, I have only named a few common ways.
Here, check this article out (http://www.gamecareerguide.com/featu...lio.php?page=1
I ran across this article a few months ago. It was very informative for me and posted right here on the site! Give it a look through and it will help break down game development and the different jobs encompassed under it.
Best of Luck!