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
(based on Kathy Shrock's guide to evaluation)