Start with your topic sentence.  I am researching adolescent healthy eating initiatives.  

Next break it down.  What are other words that can be used?

adolescent  young adult  teen* picks up teen, teens, teenager, teenagers

healthy eating  nutrition

initiatives  programs projects  studies

Connect them using the following words

and : narrows your results.  All words must be in the results  adolescent and healthy eating

or : broadens your results.  Any of the words must be in your results. healthy eating or nutrition

not : excludes words.  adolescent not (males or boys)

() : allows you to combine it all together.  (adolescent or teen* or young adult) and (healthy eating or nutrition) and (initiative or program or study or projects)