It's really impressive that you already have experience with music production and songwriting at only 15! The most important aspect to have at this stage is a good portfolio of some of the work that you have produced, such as tracks that you have solo-produced or collaborations with other artists. If you already have this set, such as by having a SoundCloud account or other online portfolio, I recommend that you look into online talent pools where you can submit demos to big labels or producers!
Depending on the type of genre(s) that you focus on, there are different labels that you should check out. For instance, if you are into electronic music (e.g. EDM, trap, dubstep, etc.) you can check out Spinnin' Records, Armada Music, Revealed Recordings or STMPD Records, for instance, all of which have online submission pages where you can upload your tracks to be heard by A&Rs at major labels! If they like what they hear, the process can be very fast and you can find yourself with a contract fairly quickly. The same applies to other labels, and you can also check out some local ones within your city if you like. In all, the more people that hear your work, the higher your chances of getting into the industry. Don't feel bad if you didn't get signed to your favourite label, or if after many attempts you find yourself still searching, as it can take some time, and these people often have a lot of people to check out. Besides, you can always try again once you get better!
Of course, there are also more passive methods, such as by posting regularly on social media platforms (e.g. Instagram), or showcasing your work at in-school events (e.g. talent shows), so do try these out as well. Social media advertising can be a very powerful tool as well, so also check this out.
I hope this helped!
Sounds like you are already in production. That's great. I can really only speak to electronic music but I guess the best recommendation would to get to learn several of the DAW ecosystems (Logic, PreSonic, Ableton, GarageBand, etc). I've been doing this since rats nest of MIDI cables but now software instruments are incredibly powerful, as you probably know. Experiment with various styles/ genres until you "find your sound". That can evolve of course but it helps with your marketing.
Definitely register with either ASCAP or BMI for royalties.
Now back to marketing you want to align yourself with other artists/ labels in your genre (target marketing) along the line of what Colin L mentioned previously. I operated a streetwear line so launching a label was pretty easy as we were selling into the same demographic (ravers and skaters, etc). We ended up forking the label into one for breaks/ electro, another for house, another for DnB/ dubstep, etc. We ended up licensing several titles to bigger labels like Tommy Boy and video games, etc so that helped us continue to grow. It was a lot of fun. Ended up opening a brick and mortar record/ dj shop in the SF Bay Area and ended up selling the business and work in Marketing for tech now.
Eric recommends the following next steps: