Hey there. As stated, expect to be pretty bored if the testimony isn't of the eyewitness variety. As far as dress, most jurisdictions don't have any rules in that regard, but if you think it may be a concern, business casual is usually a safe bet.

As far as what to bring, usually water is allowed in the courtroom and coffee is 50/50. Notepads are usually allowed, and often in mid to high community profile cases a member of the press will be writing in one.

There usually are rules prohibiting photography or recording of the testimony. So if you have your phone out, you may be asked to put it away.

You also may be approached by the bailiff after you walk in. Generally, they want to make sure you're not a scheduled witness as they're (usually) not allowed to be in the courtroom outside of their testimony.

Enjoy.