The definition of the setup is just that—it "sets up" the opening scene of your novel or story by giving a snapshot that allows what will take place in the following scene to be clear to the reader. The last think you want to happen is to force the reader to "backtrack" to make sense of what’s taking place in the scene. That’s why opening directly with dialogue is usually a mistake. Unless the dialogue is crystal clear as to who’s talking to whom and about what,, the reader may have to go back and reread the dialogue once she figures out the context and who the participants are and their relationship to each other.
Setup can take any number of forms or combinations of forms. The overall "rule" is to only give what’s absolutely necessary for the reader to understand the scene that will follow and no more. Remember, this is the beginning and the beginning is the place the reader will decide whether to in vest any more time in the story. That means little or no backstory—save that for later.