Composing music is basically the process of externalizing & documenting the music that's playing in your head/imagination... Developing a certain level of proficiency on the keyboard/piano (or the guitar if you have an aversion to the keyboard) will prove to be an extremely helpful & useful tool.
I recommend the keyboard for several reasons.
It is easier to play (it was designed to be easy & comfortable to play- the guitar requires many awkward & uncomfortable maneuvers & activities with your fingers- including the development of calluses!)
The keyboard also provides a visual interface/palate that can be very helpful in facilitating your learning & understanding the underlying mechanics or 'grammar' of music (a.k.a. music theory).
When a keyboard is connected to a computer with some basic (cheap/free) software/virtual instruments (or other sound-producing hardware) it can produce basically ANY sound there is!
I could go on listing many more reasons, but I hope that should suffice.
My advice for learning to play the keyboard is: to focus on a method/technique which emphasizes 'playing by ear'. Because, as I just said: composing is writing down the music in your head- which is just another way of saying "playing by ear"- playing what you hear in your head.
Personally, I had a unique set of circumstances that got me started when I was younger. In a nutshell: we lived in a very isolated & remote area- no potential teacher or even books to guide me. BUT we did have a piano, and I had lots of time. I first figured out how to play a couple simple melodies, gradually I started figuring out which of the other notes sounded right or good playing with the melody. I didn't realize it at the time, but essentially I was figuring out the fundamentals of music theory. Before long I learned how to play on the keyboard virtually anything I could imagine in my head, or that I would hear from the radio, etc.
Later on, when I was able to take music theory classes & piano lessons, I was able to progress quite rapidly because I was really just learning the official words, terms & techniques for the concepts I had already formulated.
Basically, the idea is for you to begin playing & 'making' music as quickly and as much as possible!!!
I would also urge you to join some kind of local community or church choir/band/orchestra (you will certainly be able to learn to play percussion well enough). A choir is really ideal- you will learn a lot about music by learning & performing it- especially in an ensemble! I think that is the most logical & 'easy' way to discover & truly understand how music comes together- bit by bit & layer by layer. That will also be an intuitive & efficient way to familiarize yourself with the basics of 'written music' & how to read it- but without the dull, complicated, abstract setting of a music theory class or some 'textbook'...
In addition to joining a choir or instrumental ensemble, I would implore you to also find a drum circle that you can go to and participate in. Most cities & even medium sized towns will have something like that which meets at least once a month if not weekly. A drum circle will be like a treasure chest of a completely different kind of information, experiences & concepts that you won't be able to find anywhere else! If nothing else, it will just help to get your creativity stimulated & flowing...
I hope this has helped to broaden your concepts & ideas about how you might go about learning to create original music. I hope I have managed to instill in you the importance of immediately "getting your feet wet" by getting involved in "making music".
