How do I put my own idea on an iron-ons and what exactly is it?

Question

I have a poem written that I want to make shirts and post it on the back.

Answer 1

Iron-on transfer paper is available for your computer from art, craft, and office supply stores. You draw the design on the computer, or scan in the design, then you print out the result on the transfer paper, iron it on your T-shirt, and you're done.

Answer 2

At our local (large) office supply business, they have papers (like copy paper) which are used for copying your 'design' onto, via running that paper through your printer, then ironing it onto your Tshirt.
Alternatively, you could take your design to a printer (of Tshirts etc) and have them run-off your design onto Tshirts.
If it's a one-off Tshirt, I'd iron it on. But if you want to mass produce it, you could use the printer people, which could be expensive-ish for you if you don't have a market for your Tshirt.
If you think it will sell, go for it and good luck.
:) :)