This is a good question. The most important part of any essay is the thesis statement. You should spend substantial time ensuring that you have a great thesis statement. Here are some characteristics of a good thesis statement.
First, a thesis statement should be debatable. This is to say that a great thesis statement must not be something that is a fact. A thesis is your point of view or opinion in the context of an argument. For example, there is a debate going on about what the EU should do with Greece. Should they allow them to fail or do something else? A thesis is your opinion.
Second, for there to be a successful thesis, you will need some good supporting points. In other words, what are your reasons for thinking your thesis is correct. The better your supporting points are, the more successful your thesis will be.
If you keep these two points in mind, then your paper will be successful.