If you really, really want to be a film composer, and you really, really want to compose for orchestra, then you have a fairly long and serious path of study ahead of you.
You could get an education at an institution, but it’s questionable whether or not it would pay off in the long run. You’d get stuck with student debt and that ain’t good. You could also as you say try to find a mentor, or possibly tutor, and that would probably be cheaper, but it might be hard to find. In addition, you would have to somehow know that the tutor is actually good at educating you.
My recommendation is to find books and videos on the topic of composing for film and consume that first to get an idea of what it entails in general. There’s a lot to learn.
Then I would recommend that you reach out in your local community and try to find film makers that would be willing to give you a piece of film with dialog and sound effects but without music for you to practice to. That way you can have a real product to work on, and once their film is done you can compare what you did with what their chosen composer did. Analyze the differences and figure out why certain choices were made.
After you’ve gotten the workflow figured out and have done a few “mock” scores on your own you can contact film makers again and offer to score a film for cheap or even free (though I always recommend getting paid!).
In all of these cases above I recommend you aim for short films. It’s maybe hard to think of this intuitively as a musician with a band and song background, but it gets waaay harder to compose something the longer it gets. It’s not like you write one song for a 5 minute movie, three for a 15 minute movie, and twelve for a 1hr movie. It doesn’t scale linearly. So starting off with short films is definitely the way to go.
I also recommend that you begin with what you know. If you are a guitar player then start off using that as your primary tool for composition. I’d recommend that instead of diving straight into orchestral perhaps you can start with a genre you’re familiar with and expand on it. There are definitely examples where you have a basic rock groove at the heart of a composition and recording, but it is embellished with strings and percussion for example. So if you can get the composition right, you can then either study smaller ensembles to add to your band-sound, or;
- you can collaborate with others that can make up for what you don’t know. If you can compose the melody and harmony and hit the right points of the movie, you can maybe find an orchestrator that can create a string score for a small ensemble for you. That way you get moving without having to know everything from the beginning.