Source evaluation is a critical part of research. You will want to evaluate your sources for many criteria including:
How well the content of the source matches your topic
Author's credentials on the topic
Author's purpose for creating the source
How the source fits into the discipline in which you are writing
How well the information in the source lines up with similar information sources
Timeliness of the resource if relevant to the topic
Whether it meets your assignment criteria
Any apparent bias that is at odds with your writing purpose
The above list is by no means exhaustive, but it gives you some guidance on questions to ask about your source. How you evaluate a resource will depend on what kind of resource it is. For example, if you use a scholarly, peer-reviewed article, the author's credentials on the topic have already been vetted for you and you can concentrate on the relevancy to your topic and whether you see any problems with their arguments. On the other hand, websites found through Google vary greatly in quality and you should start by looking for evidence that it is a credible resource.
Below are some other resources that can help you evaluate individual information resources.