Despite all the advances in web and database searching, computers still don't do well with meaning.
When you type a word into a search box, the computer looks only for the word you typed, not the concept you had in mind. To you, that word is an idea. To the computer, that word is a bunch of letters.
The keywords you choose have a direct and measurable effect on the results you get back. If you aren't finding useful sources with your current keywords, don't give up! Even a small change in your keywords can lead to a big change in results.
The first step in thinking of keywords is to define your topic. Try writing your topic down as a sentence or a question.
If you do the research process correctly, you'll discover new keywords and concepts along the way!
Example:
Childhood obesity is a growing epidemic. Should parents be held legally responsible for their child's obesity?
Concept 1: childhood obesity
Alternative terms: overweight, children, adolescent(s), adolescence, malnutrition, epidemic, health
Concept 2: prevention & legislation
Alternative terms: Health promotion, child health services, nutrition policy, role of government, regulation, legal
Concept 3: parental responsibility
Alternative terms: parents, negligence, nurturing, raising children, parenting