Don't underdress. If you have to go to a formal or semi-formal event, then you better know what that means. It's better to overdress a little than to underdress, and it's not okay to show up to a formal event wearing jeans if you needed to wear slacks or a dress. It's not funny to wear sneakers if you needed to wear dress shoes, either. Make sure you know the dress code and ask the host or your friends who are also attending if you're confused.