Its easy, take an image you enjoy and make it black and white.
Black and white means it can have NO gray, only the colors black and white. When I print the design on the shirt I can use any 1 color of ink but the design needs to be black/white to start.

You stink at design? well, I can try to help you for a little extra but i'm not great either. If you haven't tried the site Fiverr.com they can do awesome design work at a very fair price (thanks foreign artists!)

For example here is one of my designs

Here is the screen for it (I inverted the colors so it wouldn’t look odd)

Here is one of the printed shirts!

Some other designs to give you an idea

For the sonic shirt above I missed the g at the end of ring, this is too not great and I would redo the shirt for you, just FYI

That’s not David Bowie from the labyrinth, that is my brother dressed as him for Halloween and edited into black and white

Some designs to show what they look like before they are shirts and to show how black and white they must be!

It's important to note that a lot of the shirt examples above are copy right infringement and cannot be sold or made and were for my personal use only, they just show how this works.