Your teacher wants you to use active voice instead of passive voice. Active voice involves using verbs that indicate doing something rather than being something.
To write using active voice, figure out who or what is doing something in the sentence and make that person or thing the subject of the sentence. Then choose a verb that shows what they are doing.
Passive voice: The book Fahrenheit 451 is written by Ray Bradbury. (Subject=book, verb=is)
Active voice: Ray Bradbury wrote the book Fahrenheit 451. (Subject=Ray Bradbury, verb=wrote)
Avoiding be verbs in an essay can be difficult. If you're like me, avoiding the be verbs while writing your essay can distract you from the essay completely. Try writing the essay without working about how you write it. Then, go back through and circle any be verbs you find. Once you have found all the be verbs, change each sentence from passive voice to active voice. It sounds like a lot of work, but it might be easier just to get the words on paper before you worry about how they sound. Writing an essay this way is no different than proof reading before submitting a final copy. Really, you should always try to avoid using be verbs in any essay. Changing passive voice to active voice should always be part of your proof reading technique.