No matter where you're searching, you will need to pick out the most important keywords. These are generally nouns.

How can we prevent youth from becoming obese?

Other related words for prevent and obese might be:

prevent: prevention, protect, educate, education, campaign, teach, program
obese: obesity, overweight, weight, body mass index, BMI

Note that we've included variations of keywords such as verbs and nouns. Include any alternate spellings of words or abbreviations, too.