Advice on how to not humiliate myself. Details inside.

Does anyone on here know how to model on a runway? Because I have signed myself up to model prom dresses for my school's fair and I have never modeled in my life. Any advice on how to walk/pose/facial expressions?

Don't get me wrong, I'm hugely excited and I can't wait, but I am very nervous too.

The picture shows a dress very similar to the one I have to wear.