Here's a link to your thread in Legal matters www.mumsnet.com/Talk/legal_matters/1529507-help which help other responders gain an overview.
The questions you'll be asked will be to do with matters of fact such as where you were at the time, what he did to you, and what injuries you sustained.
A Youth Court is very different to an adult Court and the Judge will not allow his barrister or solicitor (the Defence) to badger you.
As I've told you, on the day or just before, the Prosecution (which is the barrister or solicitor acting for the Crown Prosecution Service) will go through your police statement and talk to you about the evidence you'll be giving and the possible questions you may be asked.
Please trust me on this: you are best advised not to anticipate what you will be asked or try and rehearse what you are going to say. All you need to do is answer all questions honestly and to the best of your ability.
If you have not heard a question clearly ask for it to be repeated and if you do not understand a question, say so.
If at any point you feel to upset to continue, don't be afraid to ask for a short break.
The questions you'll be asked will not be any more embarrassing than those you have already been asked by the police and you should remember that you're only going to need to give evidence once - and then it will be over.
Don't be antagonistic to his barrister/solicitor. Whatever questions are being asked by either side, please bear in mind that they are being asked so that the Court can establish the truth and the Judge give his/her verdict.
What you don't want to do is get into some kind of strop if you don't like the questions you're asked, or if you feel angry or upset that some of them are being asked.
Look at the camera when you're answering and try to avoid rolling your eyes, gazing up to heaven, heaving heavy sighs, pulling disapproving faces or otherwise coming over as a being something of a drama queen rather what you are - a young girl who had the misfortune to get involved in a relationship with a young male who attacked and abused her
Also, please resist the temptation to look or sound triumphant at any time. If he's found guilty you can punch the air and scream 'Yay' when you are outside of the Court premises!
As I've already said, the Youth Courts are very different to their adult equivalent. They are far more low key and everything will be done to make sure that you feel as comfortable as possible.