You need to move quickly to get a court order for genetic testing of the child and the potential fathers of the child. In WA, generally 4 years after the birth of the child, the right of the adults to challenge the parentage of the child likely expires.
If you are the biological father of the child, you will be paying child support for the child.
The mandatory pattern forms for most family law proceedings are free at http://www.courts.wa.gov/forms/ .
You will need to file a petition with enough information to support that you are potential the biological father of the child.
WA LawHelp may have enough information for you through the free self-help guides. The link is at the court's website.
Likely, you would want to hire an attorney.