Learning an instrument will make composing a whole lot easier, partially because you will learn how music notation works. It also exposes you to lots of instrumental music. As an amateur composer (and high school band member), I would suggest learning an instrument before composing. I only play two instruments, so with the limited knowledge I posses, I can give you this little tip: If you are interested in playing a saxophone, learn clarinet first. Learning to play piano would help you out a lot, but learning an instrument that is part of a concert band/orchestra will give you more knowledge in how large ensembles work.
As most composers can agree on, music theory is very important. I would suggest a book on music theory. You can do plenty with only a limited knowledge of music theory, but the more you learn, the more advanced your works will become. The reason I recommend a book is because the author will start out very basic and reveal more advanced concepts later on. This makes it very easy regarding what to learn first.
If you do start composing, don't be afraid to use repetition in your music. There's nothing wrong with repeating something you like. Just listen to this piece from Disney Pixar's Up. You should start by composing simple piano pieces. My first piece was a simple piano solo. It had no chords, a simple melody which took advantage of counterpoint (for lack of musical ideas), and plenty of repetition.
My last piece of advice is this: don't get discouraged! If you come across a part in a song that you can't play, slow down and take it note by note, playing it faster every day until you get it as good as you can. If your come to a point where your mind is completely void of musical ideas, that's fine! That's happened to me plenty of times. And it usually comes to me later. I'm no professional composer, but I've written some decent pieces. The best way to get inspiration is listening to composers that who's work you admire. some of the greatest modern day film composers are John Williams, Hans Zimmer, and Michael Giacchino.