Misunderstanding the essay prompt or basic requirements often is one of the biggest problems students have. Before sitting down to write, be sure you have read the formatting requirements and the essay question or prompt thoroughly. Ask your teacher to explain anything you find confusing. As you write your essay, look back at the assignment occasionally to make sure you are staying on topic and fulfilling the basic requirements your professor has set.
Many students have trouble understanding whom their essay should address and what sort of tone and language is appropriate for that audience. For most essays, assume your instructor is your audience (unless she tells you otherwise), which means that your essay should have a formal tone, rather than a conversational tone. In other words, avoid writing as you talk in casual conversation. Avoid using slang, first or second person ("I" or "you") and contractions.
Careless grammatical errors and typos are more serious problems than many students realize, and in some classes they can have a significant impact on your grade. While most word processing programs automatically check for common spelling and grammatical errors, there are many mistakes that these programs won't catch. If grammar isn't one of your strengths, buy a style guide or check out one from the library so you can familiarize yourself with grammar rules and review grammatical issues you find confusing. Also consider visiting your school's writing or tutoring center for help.
In many cases, problems that mean the difference between an A or a B on an essay are stylistic. To cut down on wordy and convoluted phrases, avoid using too many adjectives and adverbs, especially ones that are vague in their meaning, such as "excellent," "unique," "very" or "extremely." To avoid awkward phrasing, read your paper out loud to yourself and rework sentences that sound confusing or strange. Only use vocabulary that you understand. Don't search the thesaurus for a big fancy word just to impress your teacher, because if it isn't a word you typically use, you might use it incorrectly.
One of the biggest problems students have with essay writing is understanding plagiarism and properly using and citing sources. Be sure you understand what sources your instructor wants you to consult and avoid using other sources unless you are supposed to. Ask your teacher or professor what citation style he prefers (usually MLA, Chicago or APA) and consult the appropriate style guide for formatting rules. Most importantly, if you take any language directly from another source, use quotation marks and properly cite it. Otherwise, you have committed plagiarism, which carries severe penalties at most schools.