This is actually a two-part question:
First, in which county should I file my lawsuit?
Second, in which court should I file my lawsuit?
Let's break down both questions
You can generally file your lawsuit in one of two counties:
1. The county in which you were injured
2. The county where the defendant lives
Example: John Smith injures you in a car accident. The accident occurred in Seattle. John Smith lives in Tacoma. You can file your lawsuit in either King County (county of accident) or in Pierce County (defendant county). In legal terms, we see that venueis proper in either King County or Pierce County.
Now, of course, if John Smith also lives in Seattle, then you have no choice: you have to file your case in King County.
Note that you cannot just file your lawsuit where YOU live. Take the example above: If you live in Mason County, you can't just file in Mason County because it's more convenient for you.
Generally speaking, you can file your lawsuit in either district court or superior court. Which court depends on the issue of jurisdiction but also depend on strategic considerations.
First, jurisdiction. "Jurisdiction" means the Court has the power to hear your case, and district court has more limited jurisdiction than superior court.
To file in district court, you cannot be asking for more than $100,000 in damages. This does not include interest, costs, and attorney fees. Put another way, the jurisdictional monetary limit in district court is $100,000.
You also cannot file certain types of cases in the district. For example, cases involving slander, malicious prosecution, libel cannot be filed in district court. In other words, district court has a subject matter jurisdictional limit.
There is no jurisdictional limit in superior court, which means you must file any claim over $100,000 in superior court. But there is also no minimum monetary limit, which means you can file a claim lower than $100,000.
Why you would file a lower level claim in superior court? The discovery process is more limited in the district court, so there is less of an opportunity to gather information from the defendant. (This may not make a difference, however, for relatively straightforward cases like car accidents). Also, you are entitled to a jury of only 6 people in district court, whereas you can get a 12-person jury in superior court.
There may be other factors to consider (i.e. the judges, past jury verdicts, etc.). There is no “right” answer to where to file, so you should definitely research this choice beforehand or talk with an attorney.