First state your topic in the form of a question; that way it's easier to identify the main concepts.
Main concepts will usually be just some of the key words or search terms you use in your research.
Research Question: What health effects are associated with polluted drinking water?
It's important to think of other keywords that relate to the main concepts. Take time to brainstorm related terms.
These words are potentially good search terms and help further development our research question.
Water: rivers, lakes, reservoirs, streams, ponds
Health Effects: disease, illness, cancer, hepatitis
Pollution: pollutants, waste, oil spills, acid rain, sewage, runoff, contamination, solid waste
As you develop a research strategy, it's important to think outside-the-box. You'll be happy you brainstormed keywords when you see your research results.