When writing music, there are about exactly 14 million combinations of melody, harmony, and rhythm from one beat to the next. The problem for most of us, is choosing the right one. When we can't decide, we sit there squinting our eyes, rubbing our beards, plucking a few notes on the guitar... at least that's what I do. How do you decide where to start and what comes next? Know the Mechanics
As much as I hate to say this, and you might hate to hear it, music is just math. It's aural geometry. There is symmetry in music. Between C and C, you have Gb. For every chord, there's an opposite, an equal, and a parallel. Between every chord lies a handful of passing chords. Understanding how much functions will help you learn every possibility. You'll start to see similarities in different styles of music. Charlie Parker starts to sound like Bach. A Neopolitan VI chord is a fancier name for a tritone sub. Or is it the other way around?
Yet the magic happens in music when you stop looking at it as math. Each possibility is connected to an emotional response, and that's why you're writing the song in the first place, right? So what was the emotion you were feeling when the idea came to you? Or what are you trying to portray with the music? What are they lyrics saying?
Some would argue that there have been great songwriters and composers that just had a knack for writing, and formal learning can hinder the creative process. I disagree. Those great composers learned what we're learning, just differently. If music is your language, you ought to be constantly expanding your vocabulary until you've mastered the lexicon.
Just as great painters understand the use of color and perspective to move your eye across the canvas, or renowned authors know how to use metaphors to describe what's in their protaganists' hearts, the best composers know how to choose the right combination of melody, harmony, and rhythm to fan the flames of emotion.
I think the biggest rookie mistake is doing too much. Trying to fit too much into one spot because you can't decide what works best. I've been guilty of not knowing which chord to use, so I use option A the first time and option B on the next pass. All those songs ended up getting trashed. If I could have just decided one way or the other, the rest of the good decisions I made in the song might still be alive today.
Another mistake is forgetting about both ends of the pencil. Nothing has to be permanent, you can use the eraser. I now regularly change songs I'm writing after hearing my band play it a few times. I think that should be part of the process. The worst thing you can do is write yourself into a corner thinking you're stuck with decisions you made earlier. It's painful to think about how many times I've done this to myself, but over time it's gotten a lot easier to just scrap a mediocre idea that seemed great last night but now is leading nowhere.
Eliminate Your Choices
When you write a song, it's either because you're feeling something or want to feel something. Once you can identify that feeling you've eliminated 99% of the options and the song writes itself.