What do you do if someone threatens to beat you up?

If you are a juvenile, tell an adult, this may make you seem like a baby in short term but in long term it will help your status tremendously by not getting beat up and made fun of.

Added: If you are an adult you must be on your guard against people who threaten you. In some jurisdictions this can amount to a violation while in others it is nothing more than an exercise in 'free speech.' In short, as long as no overt effort is made to carry the threats out, it is something you simply must either ignore, or put up with. The law is not perfect and cannot protect you from ALL of life's unpleasentness.