Generally, humans are quite a judgmental species, always trying to understand others before getting a chance to really get to know them. The technical term for these mental shortcuts is "heuristics." In most cases, heuristics are quite helpful and allow you to make sense of what is going on around you. But when applying heuristics to humans, you shouldn't be so quick to judge.
The first type of heuristic is the "representativeness heuristic." This heuristic involves people judging the probability of an event based on what we have seen in similar events. The textbook gives an example of guessing someone's major based on their personal characteristics, despite the base rate of the two majors (i.e. psychology and Asian American Studies). The base rate is how common a characteristic is.
The other type of heuristic is the "availability heuristic." This essentially means we estimate the likelihood of an event based on how easy it comes to our minds. One example is estimating the number of murders in Michigan vs. the number of murders in Detroit. Astonishingly, people estimate more murders in one city in Michigan than the entire state! Here is a video that many teens can relate to.