Hi Nervous
I have to give evidence in Crown Court professionally, so I am pretty used to it.
Most courts will have a witness liaison section whose job it is to support witnesses, but they vary in how good they are. Ideally you should have the opportunity to go to the courtroom beforehand so you can get familiar with the layout.
Crown Courts all have the same basic ingredients, though the actual look and layout of it will vary according to how old or modern the building is. There will be a high bench for the judge who is in overall charge. In front of him (and it is usually a him) will be rows of desks and benches for the prosecution and defence barristers, who are the people asking witnesses questions.
You will be in the witness box which is generally to one side of the judges bench, usually nearest the door you come in by, so you shouldn't have to walk right through the court room to get to it, though this does vary.
Opposite you will usually be the jury, and they are the people you are addressing your answers to, even though the barristers are asking the questions. This feels a bit confusing to start with, but remember that ultimately it is the jury (ordinary people like you and me) who are the most important people, as they make the decisions about guilt or innocence of the accused.
Behind the barristers, facing the judge and at a right angle to you will be the dock, where the defendant/s (the accused) will sit, possibly with one or more prison officers.
There will be some other benches for the press and for the public, though they may be in a separate gallery.
Your main contact will be the clerk to the court, usually an elderly or middle-aged man (sometimes a woman) whose job it is to show you to the witness box, help you take the oath, and get you a drink of water if you feel the need (I always do -- my mouth gets very dry!)
Courts can be intimidating places, but this is largely because of the formality of the situation. In general barristers do not set out to humiliate or upset witnesses; but clearly they need to ask some fairly searching questions, which can sometimes feel like bullying. Also most barristers (men more than women IME) are quite good at that 'public-school pompous' manner, which can be intimidating if you're not used to it.
The barrister for your own side (ie prosecution or defence as applicable) will ask you questions first, and generally they will have introduced themselves to you beforehand. This shouldn't be a scary experience as they're on your side, and are just trying to give you the chance to explain your evidence. The other side's barrister will then want to ask you more questions to see whether your story is convincing, which can be a bit more stressful.
The key thing to remember is to not be rushed into answering. Consider the question, decide how you're going to answer it, give your answer and then stop talking. It's very easy to start blathering and start saying all sorts of things you didn't mean to say.
Barristers will generally encourage this by suggesting to you that you might have been mistaken, eg. "But is it possible it was someone else?"; or: "How many other people in this town have brown hair and wear jeans?" If you're sure it was X, then just say so -- don't start getting defensive and justifying your answer.
Often barristers will keep asking the same or similar questions over and over in the hope that you'll change your answer a bit don't fall for this one! If you're sure of your answer, just keep repeating it eventually they'll see it's not going anywhere and give up. Listen to politicians being interviewed on the Today programme for inspiration.
If you want more specific answers, do ask, but remember you shouldn't discuss your actual evidence with anybody beforehand.
hth