Use the bathroom facilities before you have to testify.
Tell the truth. Do not lie about anything. 
Be a polite witness. 
You do not have to bow to the judge when you enter or leave the courtroom. 
Tell the judge if something is bothering you or if you have a question during your testimony.
Wait until the whole question is asked before answering. If you don’t understand a question, ask to have it explained to you.
If you do not know the answer, do not guess. 
Answer only the question that is being asked. 
If the Crown prosecutor or the defence counsel "objects" to a question that is being asked, wait until the judge decides if you should answer.
Speak in a clear, strong voice, even if there is a microphone in front of you. In most cases, the microphone is only for recording what you say.
You may ask for some water to drink, a tissue or even a break if you feel you need one.
Do your best to describe in detail what happened to you, even if it means talking about things that you normally keep private. 
Some questions may be hard to answer or even seem unfair. 