For information about pleading guilty or not guilty and how to prepare for the court hearing see Going to court for traffic offences.
Possible defences
You may have a defence if you had to speed because of an emergency, for example if someone was critically ill in the car. The magistrate will decide if your reason is good enough. You might also have a defence if you did not speed voluntarily. For example, you had a seizure.
If you were not the driver of the car, you need to provide the name and address of the person who was driving.
The law assumes that speed detectors are accurate. If you want to argue that the detector was inaccurate, you would have to show that it was broken or improperly operated. You would need an expert to explain this in court. Otherwise you would need to have strong evidence to prove you were not speeding.
It is not a defence to say that you were running late for work or that your speedometer was broken. It is also not a defence to say that you did not know the speed limit because there were no signs.