Okay, I think everyone seems to have missed a pretty big point, namely composition software and tools for composition.

Musescore is a decent choice since it's free and not particularly lacking in features. If you google "Musescore soundfonts", you'll be able to find nice replacements for the really bad default instruments.

I'd recommend starting with fewer instruments and playing around on your own first, before heading into music theory, although don't do that for years without knowing what music theory is like I did. You should be able to make melodies you like on one instrument with some work, and possibly experiment with two voices at once. Once you get the hang of that, music theory (i.e. counterpoint, voice leading) will help out more.

The rest of it is just listening to music and copying techniques in them to have more tools in the library in your head library. The more you're aware of, the more "creative" you can be.