Do not agonize over word choice or sentence structure. You can revise and edit as you continue to work on the paper. Make sure that your working thesis is interesting and specific enough for the essay's length. Draft a preliminary outline that addresses the main points you want to cover. Review your notes, outline, and preliminary thesis statement and begin writing.
Some students find it easier to begin in the middle of the paper and work their way out.
Write, write, write . Do not worry about carefully crafting each sentence at this stage. Try to organize your ideas into coherent paragraphs that adhere to your overall framework. Leave your paper alone for a day. This will give you a new perspective on your work.