People have fields of natural talent, but that natural talent expects nothing unless you develop it. There are no real short-cuts to displaying a composer without a lot of work.
Writing a song
Same many skills, you can study the basics by studying or understanding about the problem. But there are limitations to reading the theory, and you will learn more by practicing or trying to apply what you have studied. So try composing some tiny, simple tunes, rhythms or beats, then consider some more and start putting these together within longer melodies and plays, or layer them into more complex protruding patterns or harmonies or polyphony. Concede what goes and what doesn’t, then study some more and try again.
You can learn the basics from Youtube tutorials of various techniques used by several composers and DJs. Since you know how to play Piano,purchase a MIDI Keyboard that connects with your Digital Audio Workstation and start recording the leads with different 3rd party VST Plugins and inbuilt plugins. You’ll see surprises once you start composing mixed tracks with each plugin.
Adopting for a movies
The amount of movies genres in annormous so it is possible to find your own style and be good at producing some certain kind of music for certain music genre. Firstly, it is good to compose numerous of tracks and send it to different Record Labels or Sound producers. Wither your material is good soundprodusers contact you and probably buy rights for your track.
The most important step is to show all your talent and make a long term deal with some record label.
Everything is possible for hard workers! So keep patience and moving on!