Teach your teenager to share their opinions in a respectful manner, so as to not come across as overbearing, pushy or just rude in general. Everyone is entitled to an opinion, but that does not mean a teen should be rude.
Sometimes teenagers can be shy and awkward, so teach them the importance of a proper greeting. For example, good manners call for looking someone in the eye and saying hello when being introduced. Sometimes, a handshake is in order. Teach your teenager to have a firm yet warm handshake and to answer questions politely of the person he is meeting.