Follow the writing process, whether you're writing for science, history, English, or whatever:
Prewriting: Brainstorm, research, plan, outline, thesis statement
Writing: Write first draft by starting with section II of your outline; write the introduction after the body of the paper is written; write the conclusion last.
Polishing: Revise, concentrating first on the body of the paper, then the intro, then the concl (revision = making sure ideas are logical and sequential and support your thesis); proofread (spelling, grammar, usage, etc.)
Do you have a thesis yet? Have you done your brainstorming (and maybe research) yet? Have you written an outline?
Check in the Essay & Research Paper Level.