Since starting university I've had real trouble with writing decent essays; despite trying the best I've managed to get was a low 2.1 (the rest were 2.2s)
If you’re reading this, we’re assuming that you already have a basic understanding of the SAT essay. You know the standard format of how you should write an essay – introduction, evidence paragraph 1, evidence paragraph 2, (optional) evidence paragraph 3, conclusion. You know that you should state your thesis in the introduction. All of this will get you a 5/8 as long as you develop your points enough.
Research is good, but the important thing is to use a lot of your own views, not just those of other authors. Use their points to make your own argument - make sure it doesn't just sound like a composite of what you've researched. Ditto this - I think that once you've mastered the technique of actually writing an essay, by which I mean the style and structure rather than content, you'll suddenly bump it up. I've had 2:1s on all three of mine so far, though I have three more still to get back and the feedback has been reasonably similar on each - I know how to structure it, but there is sometimes some depth lacking in actual content. So you can get a good mark with a well-structured, and only a reasonably well argued essay and a really good mark if your argument is strong as well.
Chapter 1: Discovering Ideas (3-25) offers a lot of information about beginning an essay, and "An Overview: From Subject to Essay" (22-24) provides a helpful summary of the writing process. You should become familiar with this information. This page offers additional suggestions to help you get started on an essay assignment.