A large mistake beginning writers make is that they switch between tenses. Sometimes they will write in the present tense and other times it will be in the past tense. To make it easy on yourself, always try to write in one tense. The tense you write in will depend on the topic of your essay, but typically, your writing should all be in the same tense.
A great way to check this is to underline all of your verbs and then go through each one, making sure that they are all in the same tense.
To improve your writing, make sure that you write in the same tense throughout your essay.