One of your first stops should be Bill and Melinda Gates Foundation Visitor Center. Here, you'll be able to orient yourself and get lots of useful information before you start exploring.
You'll be spoiled for sightseeing choice when in Downtown Seattle. Bill and Melinda Gates Foundation Visitor Center is a good place to start here, but there are numerous other worthy sights to explore. Space Needle, Pike Place Market, and University of Washington are close to the tourist information center and will top off your day nicely.