I can't tell you how many of my students have told me they've either started an idea and can't finish it, or they want to start but just can't get an idea going. The thing is there are always ideas floating around. In my case I always have ideas going, but where I get caught up is whether I'm plagiarizing or not, which paralyzes the process as well. We all have hang ups but I guess the only thing you can do is push forward anyway. It's just a simple fact of life. But to make things a little more comprehensive, let's look at 3 steps I take when creating a piece of music.
Picture youself as a painter. You have a canvas, paint (colors), a brush (instrument), and an idea. What can we do with these tools?
1. ROAD MAP - Personally I need to know where I'm going. I like to finish the song right off the bat, or at least make a generic outline. Don't be afraid to do this. Once you have it, then you can alter it and add all that cool stuff you hear your heroes doing later on in the process. Figure out where you want to take your progressions, bass lines, and simple drum grooves first, then move onto the second step...
2. INSTRUMENTATION - Think of the instruments as the colors you're going to use on your canvas. Now, based on the genre you need to know what instruments are needed. In this tutorial it's a rock ballad so we NEED electtric guitar, bass, and drums right out the gate. Add whatever else you want but just make sure you have the proper foundation.
3. ARRANGEMENT- Make sure things flow properly given the "colors" you've used. You should know things like what happens when you mix blue and yellow, red and white, red and blue etc. In other words, the sky is the limit but at the end of the day you're going to have to make changes to the notes being played somewhere down the line. It's all up to sticking with your goals at this point.
4. I'll add one more. FINISH THE JOB. Build your catalogue of references. Be a perfectionist. Where a lot of aspiring artists get mixed up is laziness. That feeling of "well they'll forgive that" or "they won't hear that". Be critical but not too hard on yourself at the same time. Stop at nothing to get the sound you want. HAVE FUN with the process and let the end result come naturally.