In a "fast and testimony" meeting, there are no assigned speakers or special musical selections. While the sacrament (bread and water) is still blessed and administered to the congreation, the balance of the meeting is for anyone to come forward and share testimony.
A testimony means to declare gospel truths, especially of Jesus Christ, as inspired by the Holy Ghost. Testimonies should be brief so that many people can participate.
Young children are welcome to bear testimony. It may be best for them to learn to do so at home until they can bear testimony without help from others.
Attendees are encouraged to come to the meeting fasting, if able. "Fasting" means to go without food and drink. On this day we neither eat nor drink for two consecutive meals. If we were to eat our evening meal on Saturday, then we would not eat or drink until the evening meal on Sunday.
The Savior taught us that purposeful fasting is more than just going without food and drink. We must also concentrate on spiritual matters.
We should pray when we fast, and we should fast with a purpose.