by contacting a DTG printer...
I have my own clothing label that i have designed using DGT printing..
There are different methods to print your design to a t shirt. In my opinion, the three best
DTG - Direct to Garment. This is basically a printer that prints direct to your material.
Heat Sublimation - You print with a special ink on a special printer and then heat transfer to your material
Silk Screening - for higher volume prints. Ink is applied direct to material from your screen
Each has some pro's and con's. The t-Shirt business is more involved than just buying some equipment and printing. I suggest you become a member of the tshirt forum and READ a lot. It is an excellant source of information.
Alternatively, you can design your products and have someone else print your shirts while you determine which method you like best and can save more money. Places like cafepress, vistaprint, knickknacks4less