The best way to start writing is, well, to start writing. Most professional writers claim that they don't know what they're going to say until they say it.
Writing is a process of thinking.
Often at the end of a first draft you'll realize that by putting your thoughts on paper, you've changed your own mind and shifted your position slightly. Or you'll realize that your ideas have a different focus than you thought they did. In the second draft you can try to defend this altered position or emphasize this shifted focus. Even if you can't begin drafting until you've done more research or completed an experiment, there are lots of writing exercises you can do along the way. Writing helps you find out where you're confused.