I’ve seen recently both on Critique Circle and on Amazon KDP (Kindle Direct Publishing, i.e., self published) works by teenagers who want to be writers. Even some novella-length stuff. Admirable as their ambition is, their writing is horrible. And there’s a good reason: you, young writer, are still a kid. Even if you’re eighteen or nineteen, you’re a semi-educated kid, trying to do something that requires an education. (High school is not an education, at best it gives you the foundation and tools you need to get an education) Here’s my advice:
Get an education. Preferably at a bona fide university, but lacking that, at least an education on the basics of writing, even if you have to take online classes.
Learn the craft. Writing is not an art, it’s a craft. Coming up with the story is art, the craft is being able to put it down in proper form. Not only proper grammar, punctuation, spelling, and no passive voice (ok, not much passive voice), but also proper story structure, character development, scene/sequel, and so forth. This requires, first, an education, then lots of hard work and practice.
Live a little. An eighteen-year-old can only write from the experiences of a child. Fine, write YA stuff. But you can’t because you have not done parts 1 and 2. I don’t think anyone has enough life experience to be a decent writer until they’re about forty. Of course, you don’t have to wait that long. Just learn how to write, then write about what you know (or can learn about).
If you insist on writing and publishing something before you have completed steps 1, 2, and 3, then at least have the decency to hire an editor. Have someone other than your mom and dad read it and give you feedback. Listen to them. Take their advice. Then rewrite the story until it is at least moderately readable.
If you want to be a writer, insist on a high level of professionalism from yourself. That means no typos, proper grammar, etc. Don’t put something into the world until it is immaculate, and someone who knows what they’re talking about says it is. Don’t bombard the world with crap. You get only one chance at a first impression.
I know all this because when I was your age I would have (and did) write stuff that is of the quality you are putting out–things that should be submitted to a teacher, not published for the world to see that you can’t write.
In a nutshell, learn to write, then write, then publish. In that order.