In general, use the acquired wisdom of others in doing any experiment, especially that of a teacher or whoever else is in charge. Make sure that before you do an experiment that you have read about it beforehand and know what you are doing. You are almost certainly not the first person to do an experiment, and someone already knows what is safe and what isn't. Always follow instructions, and neither do unplanned experiments nor modify the one planned -- even in doing the experiment on a larger scale. Recognize that fire, radioactive substances, hot or very cold objects, and all chemicals are potentially dangerous. Keep all equipment clean, and beware of broken glass that can be contaminated. Dispose of all hazardous substances properly.
You are the person most responsible for your safety. You can usually do far more to protect yourself from a bad incident than someone else can do to rescue you from a bad event. Report any known hazard that arises to the teacher or other person responsible for the laboratory.