To search the online databases for articles on a topic, you need to use something known as "keywords."
What are Keywords?
Keywords are any words that can be used to describe your topic.
Keywords are used individually or in combination with other keywords to search the databases.
Articles containing the keywords used will be retrieved.
How do I get Started?
Write your topic down on a sheet of paper.
Then identify and write down all words than can be used to describe your topic (like a thesaurus).
If my topic is "How vending machines contribute to obesity in children," I can use the keywords:
Vending machines, children, adolescents, obesity, overweight, nutrition, etc...