Whether you are writing for a piano, full orchestra or, and this has really been done, a vacuum cleaner, music has to start somewhere. This is the idea. One of the worst things that can happen when you are in the mood for composing is a total lack of ideas. However, if after a few minutes you still draw a complete blank, some of these techniques might help.
The Five Minute Keyboard Bash
Make a recording of yourself playing the piano or keyboard. It doesn't matter if you can't play well; you're not playing to an audience, you're just messing around. Now listen to your recording. You'll be surprised what you can find, some of the best ideas can turn up completely by accident.
Make Up Some Rules
If you don't know where to begin, a useful jump-start to your creativity is to set yourself some kind of system to work by. Examples of this common practice in modern classical music include the 12 tone system where tunes or harmonies are created by using all of the twelve notes, but only once, and use of whole tone scales which eliminate semitones in the melody to give a most interesting sound.
Make New Arrangements
Take a melody, chord sequence or tiny fragment of music tucked away in one of your old compositions, or a new one that you feel has potential. Now try to move as far as possible away from the original concept and make it work as a new variation. For instance, if you originally wrote a slow, grim march, try arranging it as a quick, light waltz. Or a fugue. Or a children's TV theme tune.
Yes - sing. In the shower, down the road, in the car, sing whatever comes to mind. If something very creative happens, sing it again, and over again. Sing it at work, learn to play it on an instrument if you can get to one... but don't miss the chance to get a good idea through singing, humming or whistling. This is much like a portable version of the 'Five Minute Keyboard Bash', only this time, you really have to be on the ball, and know a good idea when you hear one.
Don't Resort to Plagiarism
There is nothing worse than running out of ideas and so forming a song writing duo with your photocopier. You stand a good chance of getting away with it, there being lots of material to choose from. You could just lift a tune off an old abstract composer and hope that nobody will notice. But don't. It's not satisfying, and it won't make you any better at composing.
Now You're on the Way
From now on, you're pretty much on your own. The initial idea is the hardest part, the rest tends to flow easily. But don't stop at one idea, keep your composition interesting. A handy technique is to come back to a composition after a few hours of doing something else, when you're not listening to it, looking at it or playing it. If you find yourself thinking 'This sounds good', then you're on the right track. If you think 'I don't remember it sounding this generic', or 'Why do the chords sound strange to me now?' then something, perhaps something minor, has gone wrong.
It is worth doing, because when you are working on a new composition you are going through it piece by piece and you can't feel the music as a whole. If you take a break and come back and play it, it feels as if you are listening to it for the first time, just as your audience will be.
The rest is up to you. But keep working at it. Remember, if it sounds good to you, then there must be something good in there.