The main purpose of a thesis statement is to declare your theory on the topic or the solution you suggest for the research problem you are addressing. It is stated multiple times in a research paper; each time with a different purpose. Once you understand those purposes, with the help of a good example of a thesis statement for a research paper, you would be able to learn how to state your thesis effectively.
While choosing an example of a thesis statement for a research paper, remember to refer to the one which is prepared for the same type of research paper as what you are writing. For example, a thesis statement would slightly differ from an analytical statement. The subject also matters sometimes. Theses on can never be stated the way you state history paper theses. Hence, try to find a sample thesis statement which would suit you in every manner.
Thesis Statements - The Writing Center