So much historical information (along with information in general) is available on the internet that teaching with primary sources and finding answers to questions has never been easier. But how do you know what you find is accurate? A website must analyzed to understand its authorship, point of view, and other issues before you can finally conclude if it is useful or not.
Practical Steps in Evaluating Internet Resources is another useful guide, developed by librarians at Johns Hopkins University.
Stopping to look a source over may slow down your gathering of information, but you will find better information this way. Also, as you get used to looking critically at websites, you will get better at telling the good from the bad.
And of course you should have a few tried and true places for web resources that you go to regularly. A good selection of these is presented on the Historical Resources Page.
