I went to girls state! I did not have an interview. It was an amazing experience, but I want to go into politics and I am very involved in my local politics. There were a few girls that did not enjoy it because they weren't into government and politics. Be sure you're ready for that or you're taking a spot from someone who really wants to be there. It was the best time of my life and I've met some of my best friends there.
Best advise is to read up on the news local and national. Also if you are planning on applying for ROTC or to an Academy tell them. It is a veteran organization remember. Also speak up, speak clear, dress nice, no jeans look professional and look them in the eyes and give a firm handshake when introduced. Good luck
In some states, if you are not picked by your school or your local chapter, you can contact the state direct and if you are willing to pay your own way or find your own sponsors, they sometimes have extra slots available. My son paid his own way(not picked by high school), My daughter (Home schooled) talked a local bank and an attorney into sponsoring her. Our state was about $350. my guess is some might be a good bit higher.
Really ticked his high school off by the way, Guidance counselors thought they CONTROLLED the process. So go direct to state don't believe everything you are told. Again probably every state is a bit different.