At first it's best looking for people in your town or school. At 15 it will be hard to find people with a similar mindset towards music improvement. Join clubs in school, talk to music teachers, join music classes.

While looking for feedback is important, it is equally as important to collaborate with people and critically think about the music you made yourself.

It is hard to make connections online. Try making a music social media (which is difficult to do, you need to make yourself vulnerable to post your music). Maybe find subreddits (which i have trouble doing). Post music on soundcloud and dm people (only a function on laptop).

If you dm me some a little of your music I'd be glad to listen and give you some feedback.