Figure out who is telling the story or from whose point of view it is being told. The point-of-view characters often are the main characters of the story. Another way to determine who the main characters are is to pay attention to which characters face the most conflict in the story. The main characters also change because of the conflicts they overcome in the course of the story.
Once the main character is identified, the secondary or supporting characters are easier to see. The supporting characters are the sidekicks. Often they help the main character, yet the story is not about them and they will not go through changes from the conflicts they face with the main character.
Last are the minor or bit characters. These are the characters that fill in the rest of the world for the story. They do not play a part in the plot of the story. For example, an employee at the coffee shop from whom the main character orders coffee is a minor character. The minor characters are flat characters with no emotional attachment to the main character or the reader.