Just wear something not too casual if possible..If you don't have dress clothes try to wear a nice shirt without prints or designs on it. And just some pants without tears or holes and shoes(no sandals ) I mean its steak n shake so I doubt dress code is a huge deal honestly..

Taking off your shirt might be used as a scare tactic to intimidate the person you are about to fight, or it could simply be used to pump you up, and plus by taking your shirt off it keeps someone from grabbing it and ripping it or pulling it over your head lol