Start with a general topic in your area of research. E.g. Employment and retention of staff; SME accounting practices; or tourism in Asia.
Narrow your scope to a level that is appropriate for your work. A poster will require much less depth and analysis than a Ph.D. thesis, for instance. Conversely, the Ph.D. thesis may have much less breadth than a poster, since collecting data for too broad a group can be a huge challenge. E.g. surveying consumers for a product locally is feasible, whereas it would be prohibitively expensive and time-consuming to interview similar consumers globally.
On a practical note, one way to determine the scope of your statement is to assess what kind of data you have access to. Check with your supervisor, the library and department administration staff for any free or subscribed resources that can give you access to the data you require, e.g., financial data for companies, statistical data, market research reports or industry reports.
Brainstorming is one way coming up with keywords and issues for your thesis statement. Try various online tools such as Bubbl.us or guides on brainstorming techniques such as mind mapping, or lateral thinking.
Now that you have your keywords and issues, it is time to organize them. You can use structures such as a relevance tree (see this example) to map out the issues on a hierarchy and work your way towards a topic that is sufficiently specific and in-depth for your thesis.
If you have not already done so, survey existing academic literature for works that are similar to your thesis statement. Speak with your supervisor, your peers and even researchers from other universities if you know any.
Try to frame your statement in plain English, without technical terms. If you are unable to do so, you may want to review your statement again to ensure that you really understand what you want to write about. You can refer to guides that help you simplify and/or clarify your writing, such as the Plain English Campaign online guides.
The 6 steps are adapted from Chap 1 of :
Fisher, C. M. (2010). Researching and writing a dissertation : an essential guide for business students (3rd ed. ed.). Harlow, England ; New York :: Financial Times/Prentice Hall. [HSSML Books – LB2369 Fis 2010]
More tips on how to write a thesis statement here.