Carpool lane tickets are extremely common and people always ask about fighting the charges. You can fight any ticket by entering a not guilty plea and setting your case for trial. You will have your chance to present the facts and argue your case. Chances are that if you do not retain an attorney, the court will tell you to post bail, which could be a significant amount, possibly around $980 for carpool lane violations, depending on what the officer cited you for. If you fight your case, and set it for trial, there are many issues to consider, like constitutional issues such as speedy trial rights. Regardless, the court will give you a future date to appear for trial. You will have to come back to court on the trial date.
There are two common carpool violations that you might see on your ticket. It could be for being solo in the carpool lane and it could also be for crossing double yellow lines out of carpool lane. It is common for officers to cite people for both, or one or the other. Both charges are $100 base fines, plus penalty assessments, totaling about $490 each. However, only crossing the double yellows is subject to one negligent operator point, i.e., a DMV point.
There are several potential issues most clients have, some in their favor and some not. The court cannot consider whether you intended to violate the law, only whether a violation occurred. Nevertheless, the court may dismiss a case if the signage and markings were confusing or unclear to the reasonable person in the general situation. There are other ways the case could get dismissed, such as if the prosecution does not establish all of the parts of the law that need to be presented.
At trial, the police officer will testify first. His perspective of what happened, will be different than yours. Then, you can question him. Then, you can testify or not as to what happened. Finally, you can argue your case.
With an attorney, you will better know what issues to focus on, what to argue, and how to prepare you case. For example, in the past when I have won carpool trials, one way was to have video of the carpool lane where the alleged violation occurred. This showed the court that the lane was not properly marked—case dismissed. (Obvious Disclaimer: You should not drive and video, it is dangerous. Maybe drive the route while a passenger is videoing the route.)