Consider first what kind of information you are searching for, that is, what is your information need. One way of doing this is by simply writing down your thoughts on paper. Helpful questions include:
What is your research topic and how is it connected to other subjects?
What kind of information do you need: singular facts or a broader overview of the research regarding the topic?
What prior knowledge do you have?
What is your angle?
You also define the criteria that guides your information searching:
Do you need information from a certain time range?
In which language should the sources be?
What kind of material are you focusing on, e.g., do you want the sources in electronic or printed form?