However, there are techniques and strategies you can utilize to help you. One of my favorites is writing, like in a book, that is. I'll start by brainstorming topics, anything like past experiences, amazing or funny stories I've heard, what's going on in politics and how I feel about it. Then I choose one to focus on and I expand on it and give it details. The events leading up to it, what I was thinking and feeling, the after-effects of the event, whatever I can think of, I write. Basically, I write the outline of a story.
After that, I form the story, I section it up, and give it musical form. i.e. call one section verse 1, verse 2, chorus, bridge, solo, whatever. Don't feel bound to fit in a particular form, just do what makes sense to you. But make sure you have an ending, otherwise, if you're anything like me, you'll get stuck.
After that, try and express each part individually via music. This is where good theory and ear training will help out, but if you don't have that, don't worry. Without that, you'll basically be using the trial and error approach, but it will be much more successful because you now have a goal or map to guide you. In fact, you might say your outline is a blank paint-by-numbers drawing and now you just need to figure out the colors.
Anyway, this is the method I've been using lately with success, but there are hundreds, maybe thousands, of other methods. Here's some more resources to help you out:
http://www.tomhess.net/Articles/SongwritingPart1.aspx
http://www.tomhess.net/Articles/CreativityAndExpressionPart1.aspx
