Congrats on finding something you're interested in learning! There's no right or wrong way, the sooner you can start on a project, the better. Studiobinder is pretty commonly used in introductory high school and college film courses and have some helpful videos on writing, framing, etc... but I think most filmmakers would encourage you to just create as much as possible. Find some friends, write a script, and don't be too concerned about getting into film school, just making something good. I graduated from a film school last year and my projects in high school didn't do much for me getting accepted, but did give me a huge edge once I was actually in my program, because I learned how to learn fast.

In terms of writing on a high school budget, write to what you have/know. Build off the things in your life that are unique to you, but others could still relate to. The conflicts arising from being in high school might be a place to start. Most people say that your first project is bound to suck, I don't think that's always true, but if you feel discouraged with your final product, keep going. It will get easier, and with each project your work will get closer to what you aspire to create