Searching for existing systematic reviews on a topic related to your own research question can be a good place to start. These systematic reviews may provide a model for approaching your own review of the literature, including recommended search strategies and resources. The following are links to online libraries of systematic reviews and evidence documents for health and other areas of policy-making.
Sources with a broader focus for public health and sociology
Sources with a focus in clinical medicine
A thorough literature reviews begins with a well-defined research question--one that is not too broad but also, not too specific. Developing a research question will depend to some degree on your knowledge of existing research in the area, and so doing some searches of the literature will be a part of this process as well.
There are a number of question frameworks that may be useful to consider in structuring a research question. PICO is the standard framework used in clinical medicine, however you may find that some of the other frameworks will be more appropriate for your topics.
Based on your research question, you can begin to develop your search strategy and identify the databases and resources you will search.
It is a good idea to document your search process as you go along. This way, you'll be able to modify your searches when needed without repeating things you've already tried, and you can repeat your searches at a later date to find any new evidence that may have emerged.
You will encounter many types of articles and it is important to distinguish between these different categories of scholarly literature. Keep in mind the following definitions.
Peer-reviewed (or refereed): Refers to articles that have undergone a rigorous review process, often including revisions to the original manuscript, by peers in their discipline, before publication in a scholarly journal. This can include empirical studies, review articles, meta-analyses among others.
Empirical study (or primary article): An empirical study is one that aims to gain new knowledge on a topic through direct or indirect observation and research. These include quantitative or qualitative data and analysis. In science, an empirical article will often include the following sections: Introduction, Methods, Results, and Discussion.
Review article: In the scientific literature, this is a type of article that provides a synthesis of existing research on a particular topic. These are useful when you want to get an idea of a body of research that you are not yet familiar with. It differs from a systematic review in that it does not aim to capture ALL of the research on a particular topic.
Systematic review: This is a methodical and thorough literature review focused on a particular research question. It's aim is to identify and synthesize all of the scholarly research on a particular topic in an unbiased, reproducible way to provide evidence for practice and policy-making. It may involve a meta-analysis (see below).
Meta-analysis: This is a type of research study that combines or contrasts data from different independent studies in a new analysis in order to strengthen the understanding of a particular topic. There are many methods, some complex, applied to performing this type of analysis.