Take care in selecting your thesis. This is really a type of persuasive essay, but you don't want to be stuck either just repeating someone else's opinion, or citing all the same sources. Try to come up with an original thesis or take an aspect of someone's thesis and develop it. You can also take a thesis and "transplant" it into different circumstances. For example, use tools of modern economics to argue about the role of medieval guilds in the development of early European settlements. Or take a study done on children in France and try to show it is/isn't applicable to elderly Florida residents. An original thesis is the best start you can make to get a high grade in a research essay.
Your thesis is the most critical aspect of your research essay. It not only organizes the material you are presenting, it also focuses your research efforts.
Again, it's a marshalling of facts to support your argument. Make sure you have found out in any academics have made similar arguments and acknowledge them in your essay, even if you did not draw directly from them. If they said things, which don't support your argument, say why these statements are either wrong or not applicable in the circumstances.
Typically you summarize your arguments. You can also end with an example or a quote, which sums up your arguments.
What are markers looking for?
As usual, a clearly-written, well organized essay. Top marks would go to an original thesis, which showed thorough research and good writing. If you have a tired old thesis, no matter how well you write the essay, your grades will be limited.