The earliest shirt with text is probably the team uniforms worn in the 1870s. Here's an example from an archive photo at the National Baseball Hall of Fame.

Outside of sports, the first printed t-shirt was a costume in the Wizard of Oz (1939) with characters wearing a shirt that said "oz" (link to a picture of the shirt).

In terms of everyday clothing, I took a look at some of the mail order catalogues in my collection. There weren't any shirts with writing in my 1936 Eaton's catalogue. There were a couple examples in the 1953 catalogue, for children: a customizable pyjama set, and some roy rogers shirts.

Just to elaborate, do you mean like t-shirts with text or people signing shirts?