Choose an interesting idea for a thesis by organizing collected evidence, examining the relationships between related facts, and examining the significance between these relationships. Afterwards, create a thesis based on your unique interpretation of this information.
To check if the thesis is strong, verify that it answers whatever question the instructor has provided as part of the assignment. Make sure that the thesis is arguable; if no one is likely to challenge the thesis, it is likely more of a summary of known facts than the foundation for a persuasive argument. Verify that the thesis is highly specific and does not rely on relatively ambiguous terms such as "good" or "successful."
If possible, ask an outside reader to react to the thesis. If his reaction is "So what?" then it may be necessary to clarify some aspects of the issue or to connect it to a larger, more understandable issue. Similarly, if his reaction is "How?" or "Why?" it may be necessary to clarify the thesis even further.
After some or all of the essay is completed, it may be necessary to adjust the thesis to better suit what is already written. This is because the focus of the paper may have shifted while you wrote it, and if the thesis is not correspondingly adjusted, the reader thinks the essay has wandered away from the original thesis.