Mormons wear the garments after they have completed their temple endowments which is a covenant they make with God via a ceremony performed in the Mormon temple. The garments are a reminder of their covenant with God, and they also promote modesty. The garments cover the top part of your legs and they have short sleeves. They are expected to wear the garments under their regular clothing, the idea is that if their clothing covers the entire garment, the outfit is modest enough for them to be wearing.
i would start small at first. maybe pick one design that you would like and take that idea to a local screen printing shop. i would have them put some art work with your saying and see if people that you know will buy them. you may have to have a dozen or so shirts made. there are also some sites online that will produce your shirt ideas and post them for sale on their website. the first idea is what i would do to start, you will have to talk to a lawyer about protecting your ideas and an accountant about the best type of business to classify yourself as for tax and liability purposes. the t shirt market is cut throat and in my experience people are bound to try to steal your work. another option is to give who ever does your art work a free shirt or shirts and take the full idea to the printer. you may also find a starting artist who may do the design for free just for expanding their portfolio, maybe post on craigslist to search for help in the art work area. hope this gives you some starting ideas.