A Data Management Plan (DMP) is a requirement of many grant rewarding agencies stipulating how, when, and in what format data collected from grat associated research is to be presented, made assessable, and stored. The details vary tremendously between individual agencies and subdirectorates. For this reason there is no single master guide to writing a DMP - only guidelines on how to determine what agencies are asking for.
Every individual agency and subdirectorate may have particular requirements for data sharing. Many agencies have online sample templates to help. SPARC is a good overview, but you should still look at the particular granting agency requirements.