Case is filed
Case gets filed and must be served upon the defendants, usually by the Sheriff. Since some people make themselves hard to find, it is not always so easy to obtain service, sometimes taking months. Once service is effected, the defendant may appear and answer the complaint but sometimes they will file various motions to get the complaint dismissed for various reasons. So, it can take anywhere from a month to many months just to get all defendants "at issue."
Once defendants have appeared and answered (and if motions must be ruled upon or complaints amended, this takes awhile), both sides typically exchange written discovery. This is comprised of written questions seeking to obtain information or clarify issues. It can involve anything from the plaintiff's medical history to the known witnesses to the incident, to a listing of probable witnesses and their testimony expected at trial. Often this is the most crucial stage because of how important the answers are and how exacting they must be, and, on the flip side, how creative the questions may be.
After written questions come oral ones, commonly known as depositions. These can include anyone who may testify at trial, and even some who likely won't but who may have information useful to the opposition. Depositions are sworn testimony under oath asked of a witness by the opposing side's lawyer. They can take time to prerpare witnesses, and time to actually question witnesses. Often, the critical depositions are of experts each side retains as these opinions provide a window to trial strategy. Whatever is testified to at a "dep" cannot be varied from at trial without risk of impeachment, so these often are an opportunity to judge credibility of witnesses.