Hi! This is my second post; you can read about me in the introduce yourself forum. You are great and knowledgeble people.
I'll tell you what happened to me. I started "composing" the first time I sat on a piano... er excuse me... sat AT a piano... (anyway it sounded as if I had played with my butt... lol) Now being serious. I don't think you have to know theory to compose simple songs, you can come up with great melodies though, just use your ear, and experiment with your instrument. Then, you'll produce better things when you get to know theory AND HOW TO USE IT, and when you learn what your instrument (and others) is capable of.
I can tell you by personal experience, I have some friends who didn't know anything about music theory when I first met them 7 years ago, yet when they showed me their compositions I felt that all the theory I learned was just stuck in my head, without knowing how to use it, and I realized they were right in trusting their ears and heart. What they were doing was great. Then they told me they "learned" to compose that way by listening to music all the time, and by analising it. They didn't write it; they knew hundreds of things by heart. Just as Mozart said, to compose use your ears, brain, and heart. I was just trying to use my brain.
Now you'll be mostly using your ear (maybe not a very educated one right at the moment), and heart (the most powerful tool you have... but it kinda seems to depend to some extent on your other "tools", brain and ear...). Use what you got. How do you learn to ride a bike? By riding a bike. You'll fall and get hurt, but it's all part of it.
So, grab your instrument and compose, now!!!
Daniel