Please remember to bring your license the day of the ceremony!
Below are links to the local offices for you to obtain your license.
In the State of Washington, there is a 3 day waiting period from the date that you obtain your license to the date that you may get married.
So plan a head and make sure that you have it a head of time!
King County: http://www.kingcounty.gov/depts/records-licensing/recorders-office/marriage-licensing.aspx
Pierce County: https://www.co.pierce.wa.us/356/Marriage
Snohomish County: http://snohomishcountywa.gov/294/Marriage
Thurston County: http://www.co.thurston.wa.us/auditor/licensing/marriage.htm