1) First, you need a strong wish to compose music and to keep making it always. Literally saying.
2) It is highly recommended to graduate music school, it makes your music a bit more interesting and smooth. However, it is not necessary to be a professional musician, there are such intuitive programs as FL Studio or Cubase (if you already know how to play music by musical notation, the second one includes notes editor). In case, if you don't know a musical notation, then at least you need to get singing lessons, there you can learn about notes, octaves and chords, about specific things, rules and directions in music, how to listen to song, how to sing it, and how to break songs into notes.
3) Start with something simple. Piano, guitar, drums. Or just with program that may undestand and edit MIDI-format. "Mid Layer" was one of the best MIDI-editors for its time, it does work with 3D sound simulation, also has an advanced music notes editor, so you can learn notes just by placing them on a patterns line! In addition, I have almost no experience in making of chiptune melodies, I did only few tracks, and this is a separate direction in music, but you can try 8-bit editors too.
4) Listen to your favourite music or just to radio station, does not matter which one, try to catch that style of playing, that approach, analyze and fantasize in that direction. That's how I started to make music in 2006. I don't know the most of musical rules, only basic and few advanced rules, so I cannot be considered as a professional musician, but at least I have a simple style inspired by Frank Klepacki, Alexander Brandon and by few other foreign and russian composers and performers. The unbelievable fact is I had to listen closely to Red Alert OST in the late 2000s for few days, few hours per each day before I got what I needed while wearing DJ-headphones.
5) Try to make something new, experiment with different genres and instruments (personal advice of Frank Klepacki, by the way), as well as try to do remakes and don't be scary of. If your music has a success, it will improve your style of playing and music in world-wide. But there is one important thing to remember: if you and/or others don't like your music, it may possibly become a trash, so liking (thumbs up like on Facebook is unacceptable, only real sympathy) and publishing of your own creations in the Internet or showing them to your friends and other people is very important. People then will just come to you and ask for songs for their projects some day.
7) If there are only 2-3 minutes of playing, this is not always a drawback. It's just your good feeling from what did you hear and wish to continue. It really depends on melody, to be honest. But it also points at that I am not a professional musician, and I am recording melodies depending on mood and wish, that's why there are melodies which are short.
Anyway, thanks for commentary and criticism, will try to remember that. Often even experienced musicians can't tell, from which part to continue and how to continue, it is up to author.