A thesis is one or two sentences that appear at the end of your introduction that communicates to your reader the main point of your work and why they should care enough to read it. You should have a 'working thesis' before you dig into your research, but remain flexible. As you learn and understand more about your topic your thesis may evolve and that is completely normal; it's all part of the research process!
The type of thesis should match the type of project; that is, the thesis for a persuasive essay will have different requirements than one for an informative essay; all thesis statements have some characteristics in common, however.
Clearly and concisely state the main idea.
Tie your supporting discussion into the main idea.
Thesis = topic + summary of main points
State your position on a specific and debatable position.
Thesis = topic + your position + evidence to support position
Outline the facets that will be explained and discussed in the essay, or present the key points of the analysis, interpretation, or evaluation.
Thesis = topic + facets/key points
If you think of your essay as a five part outline including introduction, supporting point 1, supporting point 2, supporting point 3, and conclusion, then think of the thesis as using the same basic components: topic, point 1, point 2, point 3, and position/conclusion.