You don't need to know all the answers, but you do need to know how to find them and make people feel that you will do just that. You need to project confidence so people trust you. When it comes to dress the most important thing is being clean and pressed. You don't have to have the most expensive clothes, but look as if you took the time and care to be presentable. This tells the public that you care about them and respect them.
Surveys say that the most important standard in making a good first impression is cleanliness.
When you are dealing with the public, you must forget peer group language, jargon, and slang. Not everyone will understand, nor do they feel that you are well-educated if you use slang. It isn't a big deal to use slang like that, but it does make a difference with someone who has never met you before. Another way to project a confident image when you first meet someone is to have a firm handshake, smile and look people in the eye. You certainly don't need to crush someone's hand, but just be firm.