What is a Thesis Statement?

(from http://www.writeawriting.com/writers/thesis-statement/)

A thesis statement is the grand opening of a research study or paper or like a cherry on the top of a pastry, it is an indicator for the study, that follows. More precisely it can be defined as “the land mark” for the whole work, since it defines the target that the study intends to arrive at. A thesis statement is the hypothesis that you are going to prove through your study.

Thesis Statement = Main Idea + Body

Scholarly works need a predefined theory with few presumptions in order to proceed. You cannot prove a theory out of thin air. So, a thesis statement provides justifiable grounds to your presumption so that it can be carried for further research, to evaluate for the proposed prospects.
Once you have decided on the outline of your thesis statement, you would proceed with the next steps: introduction, background; review of literature; general design of study, description of variables, research & discussion and conclusion. The main idea of the thesis statement should be made beforehand you delve into active research but, you should write the actual thesis statement only when you have drafted the whole research paper so not to miss being in lieu with the main idea.