As a general rule, a court can force you to testify after sending you a subpoena informing you what testimony they need. There are only a few reasons someone might be excused from testifying:

• The testimony includes self incriminating evidence: The constitution gives you the right to avoid giving self-incriminating evidence under the Fifth Amendment of the Constitution. A witness can, at any time, refuse to answer a question by claiming protection under the Fifth Amendment.
• The person testifying is the defendant in a criminal case: This is an extension of the protection under the Fifth Amendment. Criminal defendants can never be forced to testify.
• The witness is married to someone involved in the case: Communication between two spouses is considered privileged by courts. This means that in most cases, you can't be forced to testify against your spouse in court.
• The witness is one party's attorney, psychotherapist, or priest: These professions require their clients to tell them everything without fear of the consequences. The court therefore gives these relationships special protection, and in most cases, communication between the two is privileged.
• The witness is not competent to testify: Some witnesses are not able to testify because their age or illness affects their ability to recall events and truthfully explain them to a jury. However, this is a very difficult threshold to meet. For example, many courts will allow a young child to testify even though children may not be the most reliable witnesses. The jury is then free to consider the witness's age when deciding whether or not to rely on her testimony.

If you do not fall into any of these categories, it may well be the case that you have to testify. However, it may be a good idea for a witness to consult with an attorney to ensure his rights and interests are protected during a proceeding. For example, sometimes witnesses are afraid to testify because they fear that someone will retaliate against them. The court may be able to take special precautions with these witnesses, such as keeping the court records secret, or refusing to allow reporters in the court room.

See FindLaw.com's articles on Trials and Subpoenas for more information.