You need to know how many colors you can work also. You do not want to create a nice design to know that you have used too many colors. You will also need to look good on a piece of paper, probably not good on a customized T-shirt. Before you dedicate yourself, you can see how it looks online. Can you ring a cheap t-shirt printing company and what they recommend?
If you are a creative person, you can get help with design and font so that it cannot be hard to read and attract from sight. Maybe your T-shirt printing company will help for fees. If you need a quick change, you want to know whether the cheap T-shirt printing company can fulfill your timeline. Will it be more expensive? Will they reduce the quality, design or number of colors?