Find out where you can meet them and find out how you can be there.
I think you could also read about other meetings with famous people and
what to expect, and what you can do to prepare yourself. If there are
things you wish to get signed, see that you have a pen that can sign on
the material, if you have gifts you want to give to the person etc.
One thing is that you remember the celebrities are also people. Most
actors, for example, are very interested in people, as stories. They
would like to know you better. Prepare something you can tell about
yourself, a little anecdote about how you became a fan, and so on.
Also, remember that actors are (usually) professionals who take their work and art seriously.
How to contact famous celebrities
How to meet a celebrity
How to meet famous people
How to get an autograph through mail