There are many schemes for evaluating websites; a simple web search will turn up dozens of pages and articles. Here is one simple method that can be used on any source of health information from print to online (and even people).
Who wrote the article or site? Do they state any qualifications? If you can't figure out anything about the author themselves, does the publisher or sponsor have any importance in the field (a government agency, a hospital, a medical school, etc.)?
Bias is simply any reason for someone to slant findings or not to state the complete picture. In health research, this is often sponsorship by a corporation who sells a product related to the research, like the manufacturer of a heart drug funding a study on the effectiveness of that drug. On the web, lots of health websites are put up by companies selling health products. Of course, they are going to put the most favorable research on their pages and not put negative research up. Looking at multiple sites, especially a mix of .com, .gov, .org, and .edu sites, is a good way of ensuring more even research.
Also, be wary of anything that bases claims of new and important findings strictly on the basis of old information. ("Pharmaceutical companies have controlled the research in this subject, so our product is better.") If it sounds like a sales pitch, it probably is (though the research behind the sales pitch may be fine; however, you shouldn't be basing your work on the sales pitch itself. Find the research.)
Especially in the health fields, how recent the research is can be very important. Try for clinical information no more than 5 years old.
Does the information you are reading match other information you have already? Be especially wary of the argument that goes, "The old way is bad, therefore our new way is good." Just because the old way does have problems does not make a particular new way any better. The new way could be just as bad, or worse! If someone is proposing something new and different, they should give clear reasons why the old information is incorrect--in ways that make sense in a scientific or practical sense--and why their new information is better.
If in doubt, ask for help. Either your professors or a librarian can help you decide if something is a good source for your purpose.