Excellent question! To start, you will want to find either a pre-written case or a case template to create your own case. Next, find players to fill in each position (prosecution, defense, co-defense, judge, witnesses, etc.). Once you have gathered the required players and found a vacant courtroom, you can begin your case!
Still finding it confusing? There are various communities in AO that organize cases regularly; you need only find and watch (or even join) a case to get an idea of how the game works.
Cases can last around 5 to 6 hours.