The best way to proof a paper is to have someone else read it for glaring grammatical, spelling, and formatting errors. If you don't have anyone who can proof your paper for you, try Paperrater.com, a free online service that provides instant feedback about your paper's style, grammar, diction, etc. Just remember that Paperrater is a machine, and as such cannot truly "read" the ideas in your paper. Even though Paperrater gives your paper a score based on its "readability," your teacher still has to check that your ideas are sound and well supported.
You will have to cut and paster your document into Paperrater.com. Because it is impossible to revise anything directly on Paperrater.com, have your document open in Microsoft Word or Google Docs to make the suggested changes. Beware that not all Paperrater.com suggestions are necessarily valid. Use your own judgment to determine whether the suggestion is a legitimate one. If following the suggestion would not improve your paper in any way, ignore it. It's just a suggestion.