There are several tricks that will make your Google search more effective.  If you're looking for a phrase such as "social media", use quotation marks. 

If you want to eliminate words you can use a minus sign before the word.

Like this: Washington  -D.C. 

The domain name .com stands for commercial -- one of the main purposes of .com sites is to sell things. Consider using only one or two relevant .com websites. You can eliminate .com sites from your search, possibly allowing better sites to come to the top. Like this:

"invasive species" -site:.com

To limit your search to one type of site, add site:.org (to get various points of view), site:.edu or site:.gov to your terms. Like this:

"invasive species" aquatic site:.gov

Or specifically to a state government agency, like this:

"new zealand mud snail" site:.wa.gov

Since government agencies are in the business of sharing accurate, unbiased information -- at least in theory--these sites look good on your bibliography.

If you limit to filetype:pdf you are more likely to find free, full-text scholarly articles.