Whether you are writing or painting or cooking or pursuing any other creative endeavor, you'll never finish if you don't start. If you convince yourself every word you write or every brush stroke you commit to canvas has to be perfect and unchangeable, you will be paralyzed and never create anything.
If you can't decide what to write, write anything that pops into your head. Write a shopping list, a letter to a friend, random phrases, the description of someone you are attracted to, anything! Just keep writing until something grabs your attention and you think, "Hey there's an idea!"
It will happen if you keep at it. It will never happen if you don't. And you need to do it every day whether you feel inspired or not. You need to get into the habit of writing. Even if you end up tossing every word you have written at the end of the day, get up the next day and do it again. Train your brain!
It isn't easy. Sitting down in front of your computer screen, or at your typewriter, or even opening your Moleskine notebook, and putting fingers to keys or pen to paper is daunting when you have absolutely no idea what you want to say, but don't let that stop you. There is only one cure for writer's block; writing.
