A simple white or colored tee works best. Get rid of anything dirty, worn, torn. If you wash your car in it or wear it to clean out your backyard it s not right for the office Get rid of anything tight. That goes for guys too. Loose the tight jeans, tank tops or anything that is suggestive or bare.
Short shorts are a no no and are in very bad taste.
Get rid of the backpack and save it for weekends. Invest in a good briefcase. Get rid of the baseball cap. Save if for the softball game. Get rid of your favorite tattoo, I m not saying that you should go to local dermatologist and get it burned off, but at least cover it.