You should send details like your age, how tall you are, a photograph, and any hobbies or skills you have.
You never know - they may need a kid who can juggle while riding a bike and singing in French!
To practise for auditions, read out loud as much as you can, so you get used to it.
If you're just trying to be an extra, what you look like and what you can do aren't really a problem.
We spoke to a few of the actors in the Harry Potter films to find out what they had to say.
James and Oliver Phelps, who play the Weasley twins, said: "Just be yourself - if you suit the part you'll get it."
Draco Malfoy actor Tom Felton recommends making the casting people laugh, so you stick in their minds.
He added: "Obviously being a good actor helps too!"