Civil procedure is a year long class in law school. There are multi-volume treastises written about various aspects of pre-trial litigation in California. There are other multi-volume treatises dealing with trials, post-trial judgment enforcement, and appeals. That is simply to give a sense of scale.
Here is a very brief roadmap of how civil cases proceed in California: one page down from volumes.
The plaintiff files a complaint that starts the case. The complaint sets forth the operative facts and legal theories for recovery. The court issues a summons. Those papers (and often other ones) must then be served on the defendants. Service of the initiating court papers is very important because it brings the defendants under the power of the court. Without proper service the court lacks the power to take action with respect to the rights of the defendants.
If the defendants are served and fail to respond in time they may be defaulted and basically locked out of the proceeding. The plaintiff may then prove up a default judgment. This is a proceeding in absentia. The defendant generally does not participate because they defaulted. If the defendant wants to get back into the case the initial fight is not about who is right, wrong or how much; instead it is about whether or not the defendant should be allowed back into the case to be heard.
If the defendant responds to the lawsuit then it can dispute the complaint or otherwise attack whether the case should go forward. The parties can engage in discovery to get documents, find out what witnesses have to say, and otherwise investigate and generate evidence to support claims and defenses. This can be a long and involved part of the case. It can also be very important in that evidence will make or break a case.
Some cases settle on their own. Courts often require the parties to go to some form of alternative dispute resolution. That is, an alternative to a trial. Typically it is mediation which is a meeting with a neutral third party who tries to help the parties reach an agreement. Sometimes it is non-binding arbitration which is a hearing like a trial where a winner is declared. But if it is not binding the decision is merely advisory. That evaluation may help with a settlement. Rejecting an award may come with consequences if one does not improve on it.
If the case does not resolve then it goes to trial. As often seen depicted on television or in movies, a trial is a proceeding where the parties put on their cases through the use of witnesses and documents. A judge or jury may decide the case. There is a winner and a loser.
And then there may be post-trial motions, appeals, and enforcement of judgment.