I am asking for any idea or starting point in order to learn composition on my own. My goal is to create my music and unlock creativity, but I do not know where to start or how to start. I know some basics of music notation and some concepts and definitions like chords, scales and melody, but do not know how to make it always sound good if there is any rule for that. Any good reference or book would be useful. I don`t want to follow any style, but I want some "freedom" to express myself musically, and that freedom has to do with knowledge also... any tips or advice ?
This might sound silly, and at least initially you might feel so while doing this, but I suggest carrying a dictaphone with you anywhere you go, also while inside your own house, or even when you go to the toilet. You will get inspiration from the most unusual items and events, and at most unusual times; maybe some territorial dispute of birds on some tree, sound of sparkling fountain in the wind, people walking, e.t.c. and you don't want to miss the moment to record your inspiration, or your own original idea, and let it resonate for a while in your head, before you listen to your recording at a later date and actually start assembling separate pieces into more coherent, complex compositions.
Don't complicate when you're recording your ideas. Hum, whistle, or sing a melody you thought of in your dictaphone as soon as possible, narrate ideas for lyrics, tap with your hands on your thighs, with fingers on trash cans, anything that wouldn't be considered offensive to other people around you. Ask permission if you deem it necessary, but never part from your dictaphone (or any other handy recording device, such as your smartphone,...) and don't let social situations distract you to the point you forget your ideas. I suggest you carry your dictaphone around with a neck strap, with the recording button easily accessible at any time.
When home, upload your recordings to your computer, split individual ideas to separate files, organize them in to you meaningfully named folders for easy access, and use them in all kinds of software that is available, converting your humming to musical notes, sample recorded sounds for backgrounds, narrated texts for lyrics, e.t.c. You will most likely use loads of different softwares for this, so I won't go into their use.
If you play any musical instruments, try your ideas on them too, see what you'll make of them when they sound different. If you don't - learn some. You don't have to become a maestro to use them for composition purposes. And - experiment! Most of what will come out might not be to your liking, but don't discard your previous ideas simply because you don't see them aesthetically or otherwise befitting to anything else you recorded for the time being. You're creating your musical playground, and you'll need as many toys to play with as you can get your hands on. Create your sandbox, where you can test them out - this will most likely be various software nowadays, but use those you're most comfortable using to evolve your ideas. Only start complicating your creative processes when you believe there's absolutely no other way left, or to apply the final layer of polish to your creation.
And perhaps most important of all - enjoy being creative and don't ever let unfavorable critique discourage you or affect your ability to express yourself through music. Impressions are just temporary, while expressions are timeless.
In the words of Neil Gaiman "Just Start".
Write a single note, then write another note. Congratulations, you have 2 notes! Now add another, and a fourth. I know this seems trivial, but a composition is not made of a giant leap from the start to half way through. It's a thousand tiny steps, modifications and backtracking.
Getting personal, this is a problem that can be overcome just by throwing notes at a page. If you have a melody or concept to follow then great! but it's not needed just to start. When I write without a start point I'll allways find that the development, or some element of what I write comes out well, and then you can store it and use it later in either another composition, or as something to centre your current composition on.
I believe this is how the pro's think because I wrote a piece start to finish without chopping and changing and at a workshop the teacher said that it seemed like an early sketch of a composition
Now on to some practical resources that are helping/have helped me.
A great book that I began my compositional theory in, because it gives rules so simple and clear, that a machine could follow them. Get Notion if you have an Ipad, Musescore if you have a PC, or use whatever instrument you're used to to play the exercises.
The limitation of this book and you'll find fairly quickly is that it restricts certain note combinations that you would nowadays completely allow, but for creating a good grounding in harmony and counterpoint in perticular I would reccommend this as the starting point a million times over. By the end you'll be able to take a bassline of whole notes and write a 4 part harmony using every note value from a semibreve to a crotchet.
Written by Tchaikovsky, This is the book I'm studying at the moment. From the very first exercise this book produces beautiful results. It talks about how to connect chords, write extended tonalities and much more. I can't praise it enough, but I've only just started so I'll say no more.
The internet is just full of astounding articles on music. While not many will delve as deep as books will, the internet allows you to answer almost everything that has been done before in music. More information is uploaded to the web every day than the entire world had during the time of Tchaikovsky, and porn aside there's no shortage of great resources. The great thing about this site is that you can explore every aspect of music writing, and I encourage you to compose until you hit a snag, and then solve it on here.
Above all, keep at it. It is not through force, but through repetition that water wears down the stone.
If you understand enough about chord relationships, you can listen to songs you like, then mimic and slightly change them. Just taking the ending to "Stairway to Heaven", an oldie but goodie, do you like the A minor, G, F chord sequence? What if it were A minor, G, C instead? Can you play it in D, such as D minor, C, Bb? Can you put your own melody over it?
I also agree with comments to keep a recording device around, and 'just start'. You record because some of the best ideas come at surprising times. Then, if you doodle on the piano, and find two chords you like together, can you put a melody over them and repeat? New age music can start that way.
Learn how harmony works. II V Is. The process of tension and resolution. Dominant chords resolving down by fifths. Cadences. That kind of stuff. I wrote a book called "Understanding and Implementing Harmony". You can check that out if you like (amazon). Or you can look at Bert Ligon's "Jazz Resources" book or Tim Richard's "Exploring Jazz Piano" book.