The first draft is never going to be perfect. It is, after all, just a draft. For the initial stage of writing a draft, your goal is to write your main idea and supporting ideas on the page. Just start writing, one word after the other, and before you know it, your first draft is ready for revision.
Remember: The end goal of your draft is to get your ideas written down and to give yourself something to start with, the draft doesn’t have to be good, it just has to be.