Man that is like asking how do you read there too many things that have to be learned before you start composing. You should at least pick up an instrument, I could recommend you the piano. It contains almost all the basic you have to learn to compose.
Another thing is learn the chords and scales like if those were you're name. Those are like the origin of everything, without that all you will compose is just some random shit.
Now if you learn all that we can talk about of getting a DAW where you can create music. A good to start with is LMNS is free and very user-friendly. FL Studio is really easy to understand and it is really powerful. You can hear some awesome orchestra songs with it. Actually I think that most of people over here use FL.
That is a really short explanation of the really basic thing of what most of people do. I could write more but I am really lazy and I got to do a song so you should check around the forum there so many information of people asking things that can help you. Oh and you can of course always ask but please something not so broad like how to compose.
Sorry for my English.