Now you want to do a photo, which is actually easier than vector in Illustrator as you do not need to trap. You just need enough resolution, and to supply a white plate. For example, if you print than onto a black shirt, you want to put a spot color of white underneath the flowers and type, then your other 4 inks for CMYK come in afterwards.
For resolution ask your screen printer for their specs. 300 dpi at actual size should be enough.
https://deluxescreenprinting.net/artwork.html#:~:text=Is%20the%20fineness%20detail%20in,for%20a%20good%20quality%20print.
in regards to white plate, your printer may do that of you. If not read up on spot colors & channels in Photoshop.
You need to talk to your printer, as you may also have to worry about halftone dot size. Depends on the quality of their screen.
https://www.youtube.com/watch?v=qooBg20wYLU