What is a research question? The research question is your object of study. It is a question, a problem worth basing your dissertation around. This is not always an easy thing to find. Certainly not at the outset, when you will be making decisions about something which is still relatively unfamiliar.
After a reasonable amount of time you should be able to explain in one (long) sentence the central question which your research addresses. In order to do this, you need to make the transition from a broad topic to specific focused research question(s). For example, focusing on how the Spanish films of the 1980s and 1990s mediate and represent the political climate of the time as the broad topic, research questions could examine how films deal with the dismantling of the ideology of the Francoist Regime ideology and with the extent to which there was a parallel transition in cinema representations of the family and gender/sexual identity.
You have started to understand your topic better, although there are lots of things that still puzzle you. This is a good sign that your topic has the necessary complexity. You begin to elaborate arguments and generate insights guided by a series of unfolding questions. From a number of possibilities, one idea gradually or suddenly emerges as the most promising one to frame and explore.
Why do you need to identify dilemmas or gaps or problems? Your research question should posit a problem or a controversial issue, one that is still under debate or has not yet been pinpointed. Your work will need to be new and interesting. The question may be based on a hunch, an insight, a contradiction, a noted gap or flaw or unquestioned assumption in your subject area.
Can you take an idea and distill it into a question? Develop filters (what?, so what?, why?, how?) so you will be able to turn your idea into a 'provisional' research question, a proposition or hypothesis that will give you a clearer focus. A question that will help you focus your research strategy. The question may lead you to discover, to explore, to explain, to describe.
If you are not ready yet, take the research question through the problems or gaps or dilemmas indicated above and generate some more ideas. The outcome may be new ideas or sub-questions to your original proposition. Use your imagination and your experience.
NB Keep your question under review! You can expect your research question(s) to evolve and change during your study. Be prepared to change direction, or develop more than one question. You will learn as time goes on and improve your question at each point and turn. You will streamline your research, your ideas, your analyses, your evidence. Also your needs will change and your skills will develop.
Have you thought of writing a potential answer to your research question? Although you are still working on a tentative or hypothetical proposal, writing a brief thesis or dissertation statement or a kind of possible answer to the research question might help you focus more. Also a thesis statement is the essential first step in writing your research proposal.
NB The transition from topic to question can be difficult. It may be useful to go through the above steps several times. Once you have identified the problems, developed a potential question and written your thesis statement, speak to your supervisor(s) for advice and comments. And be ready to run through this loop again if necessary!