The first thing you need to know about community service is that it is rarely an option for very serious crimes. It may be a stand-alone sentence, or it might be a condition of probation. In addition, community service may be a required punishment for some crimes, such as property damage. In addition, community service is always intended to benefit the community in some way.
Your community service may be closely related to the crime you committed. For example, a DUI offender might be required to help with various programs that help prevent more DUIs from being committed in the future. Someone who sprays graffiti on walls might be required to clean up acts of vandalism around town. A drug offender might be ordered to assist with substance abuse programs, helping other people get clean.
You will never be required to carry out community service that you are not physically capable of doing. For example, you will not be ordered to lift heavy objects if you are an older person or you are dealing with medical conditions.