Practice and dedication, and challenging yourself.

know wut u wanna design.. even if it seems difficult.. and attempt it.. if u come to a point where u wanna do something to it that ur not sure how, look it up online.. or ask someone on here.. and eventually, u will learn all u need to know..

a long time ago one of the better designers provided this link
maybe you'll like it too

practice, look at designs you like and try to figure out how they achieved that look/style, ask questions, and practice some more.

a picture is worth a thousand words... so learn to filter/edit idea to it's simplest form to get your idea across. if you're designing for tshirts--consider your target market.

good luck! :)

1. Study threadless designs, if that's what you're aiming for. Check out what gets printed and what doesn't
2. Get the programs. Adobe Illustrator/Photoshop/Flash are three common options, depending how you'll be doing your designs
3. Learn how to use them. Illustrator uses vectors, which gives designs a crisp and smooth feel. With vectors, you can view the image at any size and it never gets pixelated. You can trace over your design in illustrator, or freehand it in the program (with a mouse or tablet). There are other ways to go about it, too, but those are just the most common~
Photoshop uses pixels, so when you zoom it, it gets blurry. For the most part I've seen photoshoppers scan in their artwork and fix up the lines/color it in there.
4. Practice, practice, practice!

Some designs use edited photographs, too! Lots of options; you just need to find your style and go with it.

yeah practice. design all day every day for the rest of your life.