Get a copy of "The Washington State Fishing Guide" by Terry Sheely or an old copy by the same title authored by Stan Jones.

Another good one is "Foghorn Outdoors Washington Fishing" by Terry Rudnick

Best source for all of the above: www.amazon.com

Follow Mike's good tips; also West of the Cascades the "great trout fishing" is generally a lakes only affair. The streams are salmon and steelhead domain. For now, I would stick to the lakes and read the fishing reg's very carefully.