Sea-Tac International Airport is located just south of Seattle and is typically the default if you plan on flying in for a mountaineering trip on Mount Baker. There are also smaller regional airports such as Paine Field in Everett or Bellingham International Airport, which can be more convenient options but are generally more expensive.
A personal or rental car is necessary as there are no reliable shuttle options to the Mount Baker trailheads, which are in remote areas. A standard clearance 2WD vehicle is sufficient, though sections of the forest service road are dirt and may be washboarded. A NW Forest Pass or Annual American The Beautiful pass is required for parking at trailheads in Mount Baker-Snoqualmie National Forest.
Mount Baker trips often begin from Burlington, Washington since it offers convenient access to either side of the mountain. It is also possible to launch from Bellingham, Washington which is 15 minutes closer to the north side but 30 minutes further from the south side. Both towns have plentiful hotel options along with good restaurants.