The official webpage for the National Weather Service (NWS). This website presents forecasts, radar, model output, and lots of weather data! If you'd like a place to start, this is a good one.
A great site for weather forecasts, especially if you live outside, or are traveling outside of, the United States. The NWS, linked above, only covers the U.S.
Your Local News Station
Your local news station may also have a good (and more localized!) weather website, so check it out!