A good place to start is USA.gov; it is the official portal of the United States Government
To view lists of additional useful websites, mouseover the "Finding Websites" tab and from the drop-down menu click on the category that best fits your information needs, or click on the links below:
How can you tell if a web site is trustworthy or not? It's not always easy, but there are things that you can look for that will help you decide.
Library Services has a tutorial on Evaluating Information that can guide you through making good judgement calls on the information you find on the web. It takes only 10-15 minutes to complete and can save you a lot of time in the long run.