How to compose music?

As I am just beginning to learn to compose, I would like to ask that how can I start to learn composing music and how do composers start making their own music when they were just beginner? Currently I am learning some theory and I have learned quite some voice leading and such, is it enough for me to compose music or do I need to learn more theory to be able to compose? Btw, I am interested in composing piano pieces. 

My advice is to just start, see how it goes, then always try to improve.

First of all, great that you want to learn to compose!

I can share my composing advice.
When I started to compose, which is circa 2,5 years ago, I did not know anything about music theory.
I did play saxophone and I learnt to play keyboard. So, I was familiar with reading notes and chords, but harmony, form, counterpoint etc. were terms I never had heard of.

To be clear: my first compositions were garbage, but I am so glad that I wrote them.
Every 'mistake' you make, will help you with composing the next piece. Experience and doing it is the key.

I started to imitate and copy Mozart's first minuets so that I became familiar with standard forms and harmony.
Furthermore, I listened to all kinds of music.

Since you say that you already have some knowledge of theory, I think you should just start composing.
When you do not like the result, do not delete it, but look why you do not like it and what you could change so that you will like it.

Good luck!

If you wait until you know "enough," you will never start, because there is ALWAYS more to know.  And as Maarten said, you learn a lot from trying to write.  It gives you definite problems to experiment with (is this too high for an oboe?) and well-defined things to research (how do you make an ending feel like an ending?), so it helps organize your music theory reading (which otherwise can feel overwhelming, because there is so much to learn). You didn't need permission to start drawing when you were little, right?  No one expected your first crayon sketches to be "good." It was just important that you were enjoying yourself, and you slowly gained coordination.  No 6-year-old playing T-ball is "good" at baseball either.  That's not the point.  The point is to go ahead and start so you can enjoy yourself now, learn as you go, and get better over time.  The best part about learning to compose is that, unlike drawing or baseball, it doesn't take up a lot of room for art supplies, and you don't have to worry that your ineptitude is holding back a team.  It's hard to find a hobby that's less of a bother to other people, so give yourself permission to start.  Have fun and welcome to the club!

2 hours ago, pateceramics said:

If you wait until you know "enough," you will never start, because there is ALWAYS more to know.

I think this is a fantastic line of thinking.
 

25 minutes ago, Monarcheon said:

I think this is a fantastic line of thinking.
 

 

I used to teach adult visual art classes.  You have no idea how many people dinked around their studio time for YEARS because they were waiting to be "good enough" to feel like they had permission to approach a gallery, or sell at open studios, or even to just try making something they hadn't specifically been taught to do.  Surprise!  There is no local art association that's going to approach you and give you a certificate giving you permission to be an artist.  You don't get a letter from Hogwarts that you have been chosen.  Just make stuff and start sharing it with people.  

@pateceramics My students sometimes do the same. Just like practicing an instrument, you're not getting better by not doing, that's for sure.