Brand your band. Brainstorm with the members and determine which types of designs fit the image of your band. For example, if your band writes a lot of songs about the environment, you could include images of trees and the sky on your T-shirt. If your band has a subversive, humorous edge, you might want to consider designing a T-shirt with a humorous message or image. If your band has a new album out, consider putting an image from the album cover on your T-shirt.
Open a graphic design program like Adobe Illustrator or Corel Draw. If you don't have a program like this, use a simple paint program.
Choose a color for your T-shirt. White and black T-shirts are usually cheaper than colored T-shirts and are more adaptable to most outfits. Think about the kind of design you are going for. For example, it might not be the best idea to print your heavy metal band's bleeding skull design on a pink T-shirt, but it might work, surprisingly. You can always go back and change the color to fit the design.
Paste your band's logo or other image to the T-shirt design. Include the essentials. Make sure your band name is on the T-shirt in a prominent position. Don't let the appeal of an incredible design detract from the whole point of making T-shirts for your band, which is to market it and attempt to make a profit.
Go back and try different colors with your finished design.
Choose T-shirt sizes that fit your audience. If your band's audience is primarily teenage girls, make a lot of girl-size T-shirts. If your band's audience is primarily big heavy metal guys, make a lot more large and extra-large shirts.