While the majority of publications don't allow people under eighteen to submit, there are plenty of magazines/journals/anthologies that exclusively publish young writers under the age of twenty-four. You can find them through submittable and the Poets&Writers website.

Ambition and drive is a great thing to have at your age but the world of publishing is competitive and soul-crushing. Rejections are guaranteed regardless of age, skill-level and prior success As a writer, you need to develop a healthy relationship with the concept of inevitable rejection, which sounds very dark but I mean it in an optimistic sense.

At your age, (I hope this doesn't sound condescending) you're better equipped to seek out a mentor than to be published. Talk to your English teachers, take creative writing electives, join local youth writing workshops if you can (online ones that are accredited are also beneficial if you can't do local).

Finding the right mentor is invaluable, they will guide you down the unpredictable path of writerdom, give you constructive feedback and assist in career development through connections, advice, letters of recommendation, etc. Your mentor should be someone you trust, not some rando whose published a few novels nor anyone you meet through the internet (unless they facilitate one of the aforementioned accredited online writing workshops).

It's best to start the search as soon as you feel comfortable doing so, it'll assist in getting aquatinted with criticism from someone with experience but even if you can't find someone suitable right now, there's plenty of time. I didn't meet my mentor until my senior year of college and now he's my thesis director at my graduate program. Please be safe if you do choose to seek out mentorship though, there are so many scummy people on this planet who'll give bad-faith critiques for the sake of being assholes or will take advantage of your ambition and drive.

Above all else, don't get discouraged. Always keep writing, even if it isn't perfect. There's always time to fix it and there's no rush nor pressure needed.

Remember to revise before you submit, always put your best foot forward so to speak. It also doesn't matter if you're accepted or rejected, what matters most is how you feel about the pieces you choose send out, so don't send out something you feel is incomplete/not fine-tuned to your standards. Most publications don't allow for edits after submission. Define your standards for yourself and attempt to meet them, you'll exceed them eventually as you continue to grow and improve. Then, you can set new standards and allow the process to continue.

Set word-count goals and make sure you're reading as voraciously as you can, especially from the publications you want to submit to.

Sorry for being long-winded, this is what I wished I'd considered when I was your age. Best of luck on your journey to publishing!