Super_lotto - I understand how nervous you are. I will try and put your mind at rest. When you arrive at Court you will be taken to the Witness Suite by the Witness Care Team. They are generally very good and will talk you through procedure. You should also be introduced to the Prosecutor who will give you a copy of your statement to read. You will not have to sit with the general public or the defendant.
When you go into Court there will be the three magistrates (or one Judge), a court clerk, the Prosecutor, the Defence lawyer and the Defendant. There may be people in the public gallery but in the Magistrates Court this is relatively unusual.
You will be asked your name by the Prosecutor. He will then ask you to give details to the Court of what you saw, where you were etc. When the prosecutor has finished asking you questions, the Defence lawyer will cross examine you. This is not as horrid as legal dramas make it out to be. All the same, the defence will try and test your evidence with regards to the distance, any obstructions, lighting, angles. They will undoubtedly put your statement to you and try and suggest you are mistaken. You simply answer questions to the best of your recollection. Once you have given evidence you will be "released" and can then leave Court.
All you can do is tell the truth as you recollect it. There will be no ramifications on you if you don't recall something or are mistaken. There is no need to be worried and in fact the Magistrates will be grateful for the fact that you have attended to try and help them make a decision.
Whilst I can understand you are nervous, there is actually nothing to be worried about. Everyone will be kind and understanding to enable you to give the "best evidence".