Most databases allow you to use truncation (*), which allows you to search a word stem and all of its various endings.
For example, countr* would search for all results containing the words country or countries. Tour* would retrieve tour, tours, tourism, tourist, etc.
Your searches will work best in most databases if you break your topic apart and then combine the concepts with AND/OR. This is called Boolean logic and is named after a mathematician
AND - narrows your searches
Ex. Obesity AND Sugared drinks
OR - broadens your searches
Ex. sugared drinks OR sweetened beverages OR soft drink*
Most databases will have pull down boxes where you can select AND or OR.
You can also use these together by using parenthesis.
(obesity OR overweight) AND (sugared drinks OR sweetened beverage* OR soft drink* OR juice OR sports drinks)
The above in pictures
OR - gets all the info from both circles
AND - gets only the info where the circles overlap