In your kind of writing, an essay of what you would do is in copy-writing. If you're busy at school at your writing on composition, then copy the literature;be a copyist.Be a composer by copying, by jotting down notes, taking down notes and transcribe it into your exercise book by word-processor. If you can try write newspaper column,letters, then you can write essay by copy-writing in the essayist, journalist, or correspondents' sence.
Now for your essay, I suggest you pay more attention on "Phrases&Clauses" on Phrases, Noun Clauses, Adjective Clauses, Adverb Clauses, and Co-ordinate Clauses: Sentences (which is good for you)
Now for your essay, I suggest you should pay nore attention on Punctuation, Reported Speech, Voice, Spelling Rules (on spellings), Transformation, Synthesis to build up the sentences. And of course the Style; The Figures of Speech.
Now the parts of Speech should be paying more attention by you for a good Exam. result. They are Nouns, Pronouns, Adjectives, Verbs, Adverbs, Prepositions, Conjunctions&Interjections.
Note two things:-
(1)Learn the vocabulary for the topics you need to write about.
(2)Improve your writing skills.