The best way for me has been to find a group of people that also write music. You all help each other and improve with each other. Think of the group Oddfuture, they came up together and now multiple of them are big time artists.

There are actually a few music feedback subs here on reddit. Try the following: r/MusicFeedback r/IndieMusicFeedback r/WeAreTheMusicMakers has a weekly feedback thread (don't post to the mainpage) r/roastmytrack hopefully that will help a bit. If I think of any more I'll edit.

Maybe ask a few different people.

People who also make music, people who don't make music, older people, younger people.

People with the same musical taste and also people with a completely different taste.

That way you get a variety of opinions. From different points of view.

I'm 15, where am I gonna find these people :P

Go out and play them live

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.

I make electronic music

Well that does make it a bit trickier