The process of reviewing existing research allows you to fine-tune your research question and contextualize your own work. Preparing a literature review is a cyclical process. You may find that the research question you begin with evolves as you learn more about the topic.
Once you have defined your research question, focus on learning what other scholars have written on the topic.
In order to do a thorough search of the literature on the topic, define the basic criteria:
What types of sources should you consider?
Databases and journals
Books
Existing bibliographies that relate to your interest
Do I know of primary and secondary sources that I can use as a way to find other information?
What should I be aware of when looking at popular, trade, and scholarly resources?
How do I know when I am done?
A key indicator for knowing when you are done is running into the same articles and materials. With no new information being uncovered, you are likely exhausting your current search and should modify search terms or search different catalogs or databases. It is also possible that you have reached a point when you can start writing the literature review.