I attended Girls State four years ago in Texas, and it was an amazing experience! Nobody from my high school had gone before so I knew relatively nothing about the program before I went, so here are a few tips to make the most of your experience:
1. Don't be shy to meet new people. Girl's state for me was intimidating because there were over 500 girls there and all of them were amazing. They played sports and were the presidents of clubs and knew things about government that I didn't know. But they were all there for the same reason I was there, so I decided to make the most of it and get to know the girls.
2. You build a government from the ground up so think about how involved you want to be. If you want to know what it's like to run a city hall or have an official meeting, consider running for a city or county position. My sister went to Girl's State this year, became her city's mayor and loved it because she actually got to do things as a mayor. I ended up running for Supreme Court Chief Justice and the campaign and speech process was so long that I never got to actually do anything as that position once I won. It's ok not to be the Governor, but run for something and think about what you want to get out of it.
3. Remember that this is about more than just a resume thing. While Girl State looks great on your resume no matter what position you hold, and it really does boost your academy applications, that's not the mindset you want to have at all. The girls I met who held onto those thoughts were clearly not the ones people voted for because you can tell they don't care about their "city" or learning about government and the processes that make a government run. Most of the girls I met were there because they were interested in politics, and that's awesome! This is a chance for you to explore a possible career, or if you're like me, I knew I wanted to go to West Point, but I just wanted to be more aware of how people can influence government at any level.
4. We sang a lot of songs. It's not weird, and it's more of a bonding time, but just warning you.
As far as getting chosen from the other high schoolers to even attend, make sure whoever is choosing you is aware you are interested. Send some emails or meet with people in person, especially the auxiliary ladies who love to chat, and with your high school teacher or counselor. Let them know why you think this is a great program and that you can't wait to apply/attend. Participate in student government or other political clubs. Passion goes far

Good luck!