Any webinars dedicated to science that I can possibly attend as a 15 year old?( And become a speaker after some time)

TEDx talks would be a good fit. But I'm don't think that's exactly what you're looking for and I don't know the answer to that. There's a good reason for that that'll answer your question. There are many webinars and organizations that have science related webinars about Chemistry and tons of other subjects. For me, the Wyss Institute has really interesting webinars that I attend and learn a lot from. However, for these big names in the sciences, they are quite prestigious and require you to be an academic or leader in the field.

To become a speaker after some time, 2 words of advice.

1. think smaller, because there's a minimal chance you'll be a speaker at a global-scale conference. Once you have small speaking opportunities, make connections and with time (as you have 2 years until senior year) you may find yourself at a large webinar/conference. I would recommend checking with local science or chemistry based organizations. Use the internet and be patient with cold calling and emailing.

2. have something to talk about. you want to have done some interesting research or have an unique perspective to hold the attention of comparatively more learnt people -- whether that may be educators, professors, researches, or businesspeople.

Hope this helps, reply if you have any questions.