The best idea is probably not to work in CMYK at all. CMYK is for the office grade printer you have at home or work.
What the correct settings are really depends on you. But the default settings are good enough. Better make a copy of your original kra file before doing color profile conversion, so you don’t accidentally mess up your original because it will change the colors.
We have many topics about this here on the forums like these:
Printing on shirts, probably using offset printing, is another can of worms.
Long story short, let the print shop handle the conversation but you can use soft proofing to make sure it doesn’t look completely off. Best case is you can get the print shops color profile to import into Krita, but CMYK is probably close enough if you cant get your hands on their icc profile.