You will often begin with an idea such as Sustainability, but you should then develop a more focused interest in an aspect of something related to that idea, which can lead to questions/hypothesis about the topic. It is always recommend to start with a topic that you are passionate/interested about since you will be spending a lot of time searching and reading about it. Your question/hypothesis might change as you uncover information along the way, making it more specific.
Example:
These are recommended databases for searching environmental and/or ecology topics.