An essay is much more than a certain number of sentences written on certain topic. To be defined by this term it should have a central unifying idea, something that keeps all its elements together. A sentence that contains this primary idea is usually called a thesis statement – and, understandably, it is considered one of the most important parts of any essay. Which means that you should pay special attention to composing it.
A proper thesis statement should be one, two sentences at the most. In short, it should let the reader understand what your essay is about, where you stand in relation with the topic and what you are going to prove, if anything.
By definition such an element should be located in the beginning of your text: in smaller essays it will be the introduction, in larger works – the second paragraph. To place it anywhere further into the essay would defeat its purpose.
Remember that you are writing a statement – this means that it should be as concise, precise and curt as possible. Avoid vagueness and wordiness – you will have time to elaborate in the essay proper, right now your task is to give the reader a taste of what you are going to write.
This said, don’t let your thesis statement limit you. Although you should probably start an essay with writing one, it doesn’t mean it should define everything you write afterwards. If you feel that research takes you into another direction, don’t hesitate to change the thesis statement according to the changes in the essay proper – just be sure that you actually can write a decent thesis statement. If you cannot put the idea of your paper in one or two sentences, it means that either you are really bad at contractions or your work has sprawled into a disorderly mess and lost its focus. Both possibilities are bad, so work on eliminating them.
In addition to that, your personal angle should be clearly understood from the thesis. It is wrong to simply announce the topic by directly stating something like “The topic of my essay is…”. Try to reword it; if there is any controversy, make it immediately understandable what is your position. Academic writing isn’t the right medium to keep the reader intrigued – he should be interested, but you aren’t supposed to shock him with sudden revelation. If you are writing about negative consequences of global warming it wouldn’t be a great idea to suddenly reveal in the last paragraph that you don’t believe in it at all. Thesis statement is aimed at removing such possibilities and allowing the reader to get prepared to the conclusions you are about to draw from your research.
Just like with academic writing on the whole, the main feature of a good thesis statement is its directness and lack of ambiguity. It doesn’t mean you should use bland language – it means that your main goal should be the overall value of your paper as a product of science and not an immediate impression on the reader.