Think about what your design is going to represent. Maybe you are advertising your cleaning company, your rock band, or your favorite sports team. Maybe you’re using a personal illustration. The purpose of the design will determine the design.
If you are advertising a company, band, sporting team, or brand, you will likely need to focus on logo. The Nike swoosh logo, for example, is a very simple but effective design. A design for a sporting team might feature the team colors or the team’s mascot. A design for your band might focus on an image of the band or a graphic that represents the band’s style or sound.
If you are making a t-shirt to showcase a personal illustration or drawing, you will need to focus on how it will look on a t-shirt. Think about how original the illustration is and how the colors are working in the illustration.
Consider using a photo in your design. Use your own photo. You may use a picture made by someone else, but only if you have acquired the legal rights to use that image. You can also buy a stock image.