Hi Meho ~
You could take Amtrak Cascades train from Pacific Central Station to Seattle, 4 hours, scenic route, no hassle at the border. 2 trains a day, morning and late afternoon. General consensus here makes this first choice. Amtrak also runs buses in between the trains to Seattle.
Quick Shuttle is another service that goes from Vancouver to SEA airport.
Bolt Bus is another service departs from Pacific Central Station as well: https://www.boltbus.com/
You could fly from YVR. Canada Line is a 26 minute ride and a cab would cost about $30.
Edited: 3:32 pm, April 20, 2013