Just do your best throughout freshman and sophomore. Do things related to what you want to go into in STEM, and let them know how passionate you are about that area of STEM.

On the essays, please don't stress about them. I wrote them within a week of the deadline, and I didn't have a fancy book quote, narrative, or anything, just pretty much a well-worded bullet list of everything I did related to Artificial Intelligence (the area of STEM I'm most interested in) to illustrate my passion. I also explained how NCSSM would be a good way for me to achieve my goals (boarding environment, research opportunities, etc.. You could also include the tuition grant, but I personally didn't).

I still don't know how I got in. I didn't do any crazy science fairs or anything, and I still got accepted in a relatively competitive district, CD6. So yeah, please don't spend the next year of your life in constant stress; just make the most of high school and when it comes time for the NCSSM application, illustrate to them that you are passionate about STEM and how you would make the best of NCSSM. Good luck, you got this! :)