How to start writing my own composition?

Hi everyone! I am new here. My friend is a composer but I'm more of a producer and I would love to come here to study from you all. I play the keyboard. I know my intervals and basic chords, C 7th and 9th etc but i'm not classical trained.

I have a question. Do any of you write the score into a sheet first before performing/recording it? My friend doesn't record but he writes through Finale Notepad. He then render the midi and replaces the instrument using Kontakt libraries. I don't write. I normally just think of a chord progression as I place my finger on my keyboard and sitting for hours trying to find a good melody and I just click the record button inside Cubase and run with it. To me this is still consider composing. The difference is I just don't write it on the score sheet. I can't read music sheet, but I'm really good with my ears. My friend writes and he is fast! His composition are pretty good even though he doesn't record it himself. He just load the midi inside his DAW before they get replace by instruments. The performance turn out so well, while mine is so messy. lol. This year, I decided to focus more on the writing aspect, I need to get some ideas first before I can start recording. My style is Celtic or anything that is slow and not so vigorous. Is there anybody who can help me write a composition? Where do I start? And why do I need to write music into sheet when I can just think of it in my head and play the keyboard and record at the same time?

You don't absolutely have to learn to read and write sheet music but it opens up whole new worlds of musical opportunity for you if you can.  

Think of how different your life would be if you had never learned to read and write English (or any other language).  You wouldn't be able to ask questions here on this forum.  Your knowledge would be limited to the size of what you could memorize.  And it would be difficult to obtain new information to memorize from people in different cities or different countries because you wouldn't be able to research who you should ask about information you were interested in.  You would always have to talk to a person directly, and take up their time to get them to repeat information over and over and over, until you were sure you had it memorized correctly.  

Being able to read and write music is very similar.  It allows you to share your musical ideas with any other person in the world who can read music, talk about musical ideas in specific detail so you can improve your music, and learn any piece of music you want, without having to have someone sit down and teach it to you, or find you a recording.  I generally perform maybe 1,000 pages of music every single year.  And I'm not even a full-time musician.  I wouldn't be able to learn anywhere near that much music if I had to memorize every note.  But I don't have to memorize, I just read it.  The same way I can sit down and read a novel.  And I can share the music I write with people all over the world.  No one would ever perform the music I write if all the performers had to memorize all their notes from a recording I made.  But since they can all read sheet music, like I can, one person can find a copy of a piece I wrote, decide it looks interesting, and send a copy to everyone else in the group to read over and practice on their own.  And at the concert, they don't have to have my piece memorized, they can still be performing by reading.  

The one quality that all the professional working musicians I know have in common, is that they can sight read sheet music, accurately, and at the proper tempo, the first time someone hands them a piece of music.  The conductor of an orchestra doesn't have time to sit down with each player and teach them their part.  There are too many players and too many parts and too many hours of music to learn.  They all have to be able to read it on their own.  That way rehearsal time is spent deciding how loud the loud bit should be, or how a tempo change will be handled... all the things that make music feel musical, instead of just teaching notes.  

(:  

And as for how to start, plenty of people write by sitting at a keyboard.  There's nothing wrong with that.  Just take the time to carefully write down what notes you are playing as you go on a computer program like Finale, or with a pencil and paper.  

So you don't memorize the notes, you just read it? That is so awesome and sound very complicated. LOL!

I watch some Finale tutorials on Youtube but it seems so boring to me. You have to click the note every time. Why do I have to do this when I can just play the keyboard?

I did see the arrangement view where you go into each instrument and draw their part. But can't you just memorize it while you are recording say inside of Cubase? See, I record and I play at the same time. To me that's consider composing too. Eventhough I don't write it on the sheet. I would give Finale a try and see if I like it. I dont even know what I suppose to do with Finale after I write all the note. I have always record on my own in real-time. I dont know why I need the sheet for. [thinking hard]

I don't get your problem and/or question. You clearly already compose, so what you want is to learn how to read music notation? ...Or what?  Are you having difficulties composing the music you want to compose?