Can you use internet sources? Heck yes! Do you need to evaluate them as much as you evaluate print sources? O YES. Not all sources are created equal. What do you trust more: information from the American Cancer Society or a link on cancer your cousin posted on facebook?
For more thoughts, peruse this.
There are certain things to take into consideration when you are looking at a resource.
Who wrote it?
Is this person qualified to speak on the subject?
What biases does this person have on the subject?
When was it written?
For most subjects, the more recent the work, the better. New developments may outdate works very quickly if the field is constantly moving and developing.
Where was it written?
Popular journals are not as stringent about accuracy as academic journals.
What sources does it use?
Does it cite other people?
What type of resources does it use?
Is it citing blogs, popular journals or academic sources?