"Walk right in, sit right down," pretty much covers the process of attending a court trial. Almost all court proceedings in this country are open to the public. You may be barred from celebrity trials, where the courtroom is full to the gills, but you should be able to watch and observe the vast majority of court proceedings that strike your fancy.
Why Attend a Trial
You might want to attend a trial for any of a variety of reasons. Perhaps you know one of the parties or attorneys. Maybe the subject of a particular case fascinates you, or you are planning on attending law school or writing a novel with a courtroom scene. Whatever the reason, it's likely you can get your wish. Most court proceedings are open to the public, including federal and state civil and criminal trials and bankruptcy trials. Just walk in and take a seat in one of the rows. The court officials won't even ask you why you are attending or whether you have an interest in the outcome.
Closed Court Trials
In a high profile or celebrity trial, there may be more people who want to watch than there are seats. Generally, some spots are reserved for family, some for the press, so you may find yourself excluded. In other cases, a judge may close the courtroom to the public for matters of security, such as to allow a secret informant to testify. If one of the parties has an overriding interest in privacy, the court may close the courtroom to the public as long as the closure is narrowly tailored to protect that interest. For example, the court may exclude the public if a child is testifying about abuse or violence.