You can drop an entire letter grade for some typical first version errors, so here is a list of some of them, why you make them, and what to do to avoid them. Ideally, you should leave enough time to check your paper for these kinds of errors before you submit it to your instructor.
During the time you spend drafting, your brain is thinking, trying to pull together many ideas. As it does so, it is not uncommon that it repeats words or elaborates by including two verbs instead of just one. Before you turn in that first version, eliminate as much repetition as you can—choose the best verb and put in synonyms when needed.
Many writers try to capture their ideas as quickly as possible on a first version, and that means they use informal language or the language of their childhood (which might be a foreign language or a dialect). Informality can convey a lack of seriousness or a lack of understanding of the subject, which is dangerous for your grade! Before you submit the paper, consider your vocabulary and read the paper aloud for any fragments or any syntax that is not appropriate for American edited English.
Watch that the tense is the same throughout; be sure of your citation style.