1. Just start composing. Don't worry about whether it's "right" or "good," just start putting ideas on paper. If you wait until you know enough to start, you'll never start, because there is always more to know.
2. Start sharing what you write. The faster you start getting feedback, the faster you can grow from that feedback. If you wait until your work is good enough to share, you'll never start sharing.
3. Give other people feedback on what they compose. Beginner work follows patterns that make it less strong than it could be. I won't say they are "mistakes," because every art form is so subjective, it's hard to ever point at any one decision the creator made and say it was "wrong," but there are things all beginning composers seem to do that make the work less impactful than it could have been. Instead of highlighting your audience's enjoyment, they undercut it. It's hard to see the problems in your own work, because you're too close to it. It's hard to accept it when people point the problems out to you for the same reason. But if you look at enough beginner pieces by other composing students, you'll start to see the patterns for yourself and notice what beginner pieces all seem to have in common, and it becomes easier to appreciate the advice others give you and avoid or fix issues in your own work.
4. Start reading about theory and orchestration. Read musical analysis of famous works by famous composers and also the feedback given by and to other composing students here. There is a lifetime of material to learn, so there's probably no wrong place to start reading. Just start and let each question that comes up lead you down a rabbit hole to new reading. What you read will inform what you want to try with your latest composing project. (Hmmm... cadences... right, let's see what this sounds like if I end it with an authentic cadence...) And sometimes what you are composing will inform your reading. (This doesn't seem to be a major scale or a minor scale, but it certainly sounds like it is a something. Maybe I should do some reading about jazz scales and it will turn out to be one of them).
5. When you listen to music, or play something for your piano lessons, start thinking about what parts you like and why, what parts you don't like, and why, and how the piece is structured on a large scale, so you can apply those structures or those things you like to your own work.
Welcome to the club! We are all here because we are learning too!