DEMOCRATS
Caucuses are happening at SCCC and the Capitol Hill Library Branch... to name a few.
Check here for your specific listing.
The Dems have this YouTube video out with more info.
REPUBLICANS
You can figure out where to go here. Or just vote in the Primary... the WA Republicans allocate 49% of their delegates at the National Convention to the Primary winner.
No fancy YouTube video... but here's a PDF.