Generally speaking, you will be asked to swear or affirm that you are going to tell the truth, and that you understand the consequences of not telling the truth. Whether you do a whole ceremony about it or not, it doesn't really matter -- but the court will want to know that you are competent to testify truthfully and that you know that you're not allowed to testify to things you know aren't true.

If you're asking "can you be forced to testify?", the answer is "Yes but it depends." If you're competent to testify and the officers of the court deem your testimony important, they can subpoena your testimony. If you have a reason to contest it, you can -- but "I don't want to" isn't good enough.