Relax and listen carefully to the questions asked. Most people get nervous about testifying in court. There are many reasons for that. It is a very intimidating environment. If you are not truthful on the stand, you could be held liable for perjury. Also, in all likelihood, your testimony is going to make someone upset. The key to testifying is really to stay calm and relaxed. Do not feel rushed. Attorneys are notorious for asking confusing questions to try to throw you off track. If you don't understand the question, don't answer it. Ask for clarification. Take deep breaths and slow the process down if you need to.
Do not Volunteer Information or Guess. Answer only the questions that are asked by opposing counsel. Only testify regarding things you personally have knowledge about. Do not speculate or offer your opinion about something unless specifically asked. And if you don't know the answer to the question, the answer is "I don't know". If you feel pressured to give an answer and make something up, that can come back to haunt you later. If the questions are asked by your own attorney, then the questions may be more open-ended and require you to give longer answers. Your attorney should be preparing you for your testimony before trial. If you are just a witness, however, you will probably not have an attorney to prepare you.
Prepare for court. In all likelihood, you will know what you are going to be testifying about. It is important that you review all relevant documents ahead of time. If you kept a calendar of important dates, bring it. The wisest choice may be to make a cheat sheet to bring to court with you of all of the relevant dates and events in the case. That way if you get on the stand and get flustered, you can use your cheat sheet to jog your memory. Most courts will allow this, although you may be required to give a copy of your cheat sheet to the other side, or it may get entered into evidence.
Dress Appropriately and Keep your Cool. You may not realize it, but part of your testimony on the stand is your personal appearance and demeanor. Both Judges and juries will be judging your credibility. It is important to show respect for the seriousness of court proceedings by dressing appropriately. Most courts have specific rules regarding a dress code, but even if they don't, it is inappropriate to wear things like tank tops, muscle shirts, and shorts. The best way to approach your clothing is to dress like you were heading to a business meeting or to church. Regarding your credibility and appearance, it is also inappropriate to use foul language in court. If you lose your temper or argue with attorneys, your testimony will lose its power. The way you present yourself in court is important at all times, even when you are not testifying.