Near the Google search bar you will see a "Settings" link. Click on it and choose "Advanced Search" from the drop down menu:
On the following search screen, you can choose to have Google search only certain domains, such as .org, or .edu. While .org can indicate legitimate and non-legitimate sources, and .edu might contain the work of students rather than educators, they are generally more reliable than other domains, such as .com and .net. Anytime you use information from on the internet, you need to determine the purpose of the website that is responsible for the page you are citing, and whether the information is written by experts or scholars.