Start here
I get this question by email most weeks. Here's the answer I actually send.
First, one question: can you read music?
Nothing extreme. Do you know the note names in treble and bass clef, and do you have some theory?
If no, start with The Staff Speaks. It's the bridge, and it builds notation and note-hearing from zero. Come back here after.
If yes, start with Flawless Ear Training.
The path
Start with Flawless Ear Training. It's going to feel simple at first, but it's fixing old workarounds and habits that prevent you from audiating, and from knowing what you're audiating. It's a lower cost entry as well.
After about a month, I'd suggest the Composing Hour if you like my teaching style.
That's basically it. Those two things combined last at least one year.
After that
If you finish the training and want a more theoretical background, there is Counterpoint for Composers and Form and Analysis. Though I recommend you work on audiation first.
Still not sure where you fit? Write to me. I'll ask you that same question about reading music and tell you where to start. I read every one.