Limiting your search to specific domains (.edu or .gov) can remove some of the less reputable websites from your Google searches.
Use Google Scholar to search for academic articles. Please keep in mind that some of the material you find may not be fully accessible.
When searching Google, you can click on the Settings link in the lower right corner to access the "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.
There are many other resources to evaluate web pages. One of the more interesting examples is CSU Chico's CRAAP Test.
(based upon Kathy Schrock's guide)