The first thing you need to do is to find your inspiration!It is not so hard!Your inspiration can be a slogan or any other object which you can image.
2.Pick up the design of t shirt
Cause you already have the inspiration,The second step is you need to find the latest style of t shirt you may like.Maybe you can pick up the style from one of the china t shirts manufacturer and ask them to add your inspiration into the t shirts and create a mockup.
3.Turn it into real products
Then you can ask the factory to manufacture based on the mockup.This is completly simple which may help you a lot even though you are new in this field.