The question

hello, I have a question about starting my own business.. I would like to start a Christian T-shirt design business.. I bought the printer.. And I bought the inkjet transfer paper to iron on my own designs.. My question is this if I use my own designs and no one else’s designs can I print on T-shirts that I buy from the Salvation Army?? Or do the T-shirts have to be non-copyrighted or something along those lines?? How do I find out what T-shirts I can and cannot print time to resell them