Essentially this is the question you are going to attempt to answer in your dissertation. So, ideally it will be a question you actually care about answering. It should also be a question with quite a narrow focus. For example, the question “What’s the best way to learn English?’ might be too wide, but the question “What factors can help 4-5 year olds speak English with more intelligible pronunciation?” might be more realistic.
There is lots of help online:
The Writing Centre takes you through the basics, as does this guide from a United States university. Queensland University have a guide here that explains how your research question fits into the dissertation as a whole.
There’s a useful video here at the University of Birmingham.
And why not practise your academic reading at the same time – try this guide from Yale University!
There are other useful links to help with your academic study here: www.DoitinEnglish.com/study