"My teachers keep saying that my writing needs more structure. What do they mean by this and how can I structure my essays?"

In English teacher speak, structure is the natural flow and progression of your argument. In plain words, structure means repeating your key ideas in every paragraph to keep yourself focused on the question. It's less complicated than it sounds and you see it all the time. Take any pop song (or rock, if that's your taste), and you'll notice that almost all of them have a chorus. The chorus is a repetition of the key ideas of the song. By repeating the chorus, the song is structured around its central idea. Take Paint it Black by the Rolling stones. The song is about Vietnam soliders returning from the war with post traumatic stress disorder and how their perception of the world has changed. By repeating the chorus 'Paint it black', Jagger reinforces the solider's depression in a colourless world. The song is therefore structured around its central idea. Look out for it yourself. You'll find the same structure in everything from Nicky Minaj to The Beatles. 

The same structure works with English Literature essays. The 'chorus' of an English Literature essay should always be a response to the title. Instead of Vietnam war veterans, the essay is about Othello as a tragic hero. If you agree that Othello is a tragic hero, then by the end of each paragraph you should justify why your point proves that Othello brought his own downfall. The 'chorus' ensures that you do not meander off-topic. If every paragraph manages to directly address the title, your essay is guarenteed to be structured around your central idea.