If you've decided that you don't like or trust someone before you've actually met them, you're prejudiced. It's unfortunately common to have prejudiced views of people who are different from you.
It's prejudiced to have negative opinions about people because of their gender, religion, skin color, or the language they speak. It's equally prejudiced to say things like "She can't be very smart — her family is poor" or "They are so old, there's no way they'll think this movie is funny." Each of these statements reveals an unfair, preconceived idea of what someone is like based on one personal characteristic.