An important first step in scholarly research is limiting the scope of your research by deciding on a topic. In order to find information, you have to know what you're really looking for. One or two words, like "public health" won't be helpful. This worksheet on Identifying Key Concepts will help you think about synonyms, broader and narrower terms, and terms to exclude.
You may also want to use the MeSH Database to explore and visualize relationships among terms.
For more focused results, these are the most frequently used databses for researching global health: