Silk screen printing is standard for doing short runs of a t shirt (say 1,000 or less) but can also be used if you're keeping a design on the market and just printing a batch up now and then.
Draw it, put it on t shirt transfer sheet (sold in box of 12) heat transfer it to t shirt.
Get silk screen. ink, brayer (roller), block out each color with a paper stencil, silk screen each shirt, let dry and repeat for each color. Be sure to work out some registry marks so each stencil lines up.