Begin writing your paper as soon as possible. The earlier you begin this process, the better your paper is going to be. It gives you the opportunity to work through many different possible topics to find the one best suited to your interests. Also, it allows you to find proper research to support your main thesis or objective in the paper.
Determine the layout for your paper and write as you are able to. You do not necessarily have to write in the order that your paper will be read. Sections that you write can be stitched together in a logical order later, so don't skip out on writing a section simply because the one before it isn't done. You can edit, delete or add information later, but until your draft is done, just keep writing. Once you have established a basic draft, you are then ready to proceed with writing the final version that will be handed in to your teacher for grading.
Remember your audience. Depending upon whether your paper is going to be read by more than one professor, adjust your writing style to your audience. Try to place yourself in the reader's shoes and see what would keep his interest while reading. Each type of audience is different, so it is important to keep this in mind while writing.
Edit your paper for spelling and grammar mistakes thoroughly, and don't be surprised if it seems to take an inordinately long time -- editing often takes as long as the actual writing does. Be sure to follow AP or MLA guidelines and keep the appropriate writing manual handy for any questions regarding format and research citations in the body of the text of the paper.