How to prepare for a graduate school fair?
A grad school fair is not quite like a job fair, where many people bring CVs and hope to get interviews---no one is expecting you to apply to a grad school at a fair.  It is a place for you to go to learn about grad schools: what programs are out there, what they are like.

As for how you should prepare: think some about what you might like to study and what things would be important to you in choosing a grad school.  For this, it may be helpful to first try to find general advice on choosing grad schools.  Then you can ask appropriate questions to each program.