If you are being married in Washington State, you must apply for and secure a valid Washington State Marriage License.
The steps are fairly easy:
1) Together, you may visit any County office in Washington State at least 3 working days before your wedding ceremony. There is a 3-day waiting period from the date your marriage license is issued. The fee varies by County and is most likely in the $64 range. You may apply in person or may start the process by mail. Also, your license is good to be used in any County in Washington State, no matter what County is was issued in.
Please see specific County websites for more information. For your convenience, here is King County, Pierce County and Snohomish County
King County: http://www.kingcounty.gov/courts/marriage.aspx
Pierce County: http://www.co.pierce.wa.us/pc/abtus/ourorg/aud/marriage.htm