Limiting your search to specific domains (.edu or .gov) can remove some of the less reputable websites from your Google searches.
.com - commercial or business
.edu - educational institution
.org - organization (often non-profit)
.gov - government organization
When searching Google, you can click on the little gear symbol to access "Advanced Search." You can use Advanced Search functions to eliminate some of the "trash" from your search results.
Try these Google Advanced Search Tips and Tricks
Whenever you are considering using a website as a source, it is very important that you review the contents in detail. An easy way to make a determination is to use Jim Kapoun's Five Criteria for Evaluating Web Pages:
There are many other resources to evaluate web pages. One of the more interesting examples is CSU Chico's CRAAP Test.