The PICO method is a good place to start when coming up with search terms.  Remember, the population includes group descriptions (eg. age, gender, nationality) and diagnosis.  It's good to start with just your population (P) and the intervention (I) you would like to learn more about.

Searching databases sounds intimidating, but it's actually pretty easy.  It's similar to searching google, except that google results show you all sorts of websites, while scholarly databases only show you carefully chosen research articles.  They're just different tools for different information needs.  Check out the comparison below.  

Start with a more general search.  You can always narrow your search to get more specific information.  

Try using only 2 or 3 words to start.  Try just your population (P) and your intervention (I), or perhaps the diagnosis and the intervention.

For example, try the terms babies and PICCs, or the terms diabetes and diet.

You can always drop words such as 'does' 'if' 'when' 'among.'  These types of words will be in every research question, so they won't distinguish what you're looking from anything else.  

The goal is to come up keywords that can get you exactly what you're looking for.  Full sentences won't get you good results in these databases.