Research the topic. Go online, head to the library, or search an academic database. You may ask a reference librarian.
Know which sources are acceptable to your teacher.
Does your teacher want a certain number of primary sources and secondary sources?
Can you use Wikipedia? Wikipedia is often a good starting point for learning about a topic, but many teachers won't let you cite it because they want you to find more authoritative sources.
Take detailed notes, keeping track of which facts come from which sources. Write down your sources in the correct citation format so that you don't have to go back and look them up again later.
Never ignore facts and claims that seem to disprove your original idea or claim. A good essay writer either includes the contrary evidence and shows why suc