How does Mock Trial work?

Hi, I'm interested in my high school's mock trial team but haven't had the confidence till now (I'm entering my junior year). I honestly don't have the slightest clue how the process works. Is it like a drama audition where you get a script and read it? Do you tryout for specific positions? If I wanted to be witness how would that work. I really need info and any would be greatly appreciated:)