Before you can start writing a coherent essay, you must carry out effective research into the topic you're writing about. You should see the research as the foundation of the essay and allow more time for it than the writing itself. Read over any notes you've made in class as a starting point and use reading lists your tutor has given you to find suggestions for further research. Make notes as you read, mark important pages in books and bookmark relevant websites so you can easily find the material you want when you're writing.
Before you start writing, you need to make a plan for your essay. If you want, you can alter it later on, but you should have a basic structure before you begin writing. How you do this is a matter of preference and depends on your learning style. If you're comfortable working visually, then a diagram or mind map can form your plan. If you think more literally, create a written outline. Whichever way you choose to do it, you need to make sure you have identified all the key issues and worked out how to use them in an argument.
Use your plan as the basis of your writing and flesh out each point with solid examples from your research. Your essay needs an introduction, where you let the reader know what you will be talking about. It then needs a body, where you make your argument by constructing logical, well-thought-out points that are backed by evidence. Provide footnotes for your evidence and use quotes in the text to illustrate your points. Make sure that the body builds toward and supports the last part of the essay, your conclusion.
The conclusion is the most important part of your essay. This is where you draw together all the different arguments you have constructed in the rest of your essay. This is your chance to prove that you can provide an original contribution to your subject. Make sure that you do not repeat conclusions you have read, but make your own based on the evidence you have provided. Be as brief as you can while still getting the point across --- superfluous language detracts from the point you are trying to make.