First make sure that you proofread your essay after writing it. Many students make the mistake of just going through it immediately after they have finished writing. Keep the complete essay aside first so to give your mind some rest. The second time you come back to it, you will look at it with fresh eyes and it will be easier to spot the mistakes. Besides checking for grammar ensure your essay is structured in the right way, has a logical flow, answers the question asked and is properly formatted. If you are unable to do it on your own we can help you.