Proofread, proofread, then proofread again.
When you think you’re done with that final draft, run it through the spellcheck on your computer, then do not read your essay for a few days. When you read it again with a cold eye, you’ll be more apt to spot typos and awkward grammar. After that, ask a teacher, parent, or college student to give it a quick read. (And, while you’re at it, double check your word count too.)