A data management plan (DMP) is often required as part of a funding application. Even if you are not required to produce one, or you are not putting in an application to a funding agency to start your project, writing one is an excellent way to identify problems and plan around them. There is no set format as to what a data management plan has to contain. Different organisations use their own formats. If you are asked by a specific organisation to produce a DMP always use their recommended template if they have one. Alternatively, if you do not have a specific template to follow then UCL provide a template data management plan template which is available here:
There is also a Moodle module which covers the data management plan process and provides an overview of the information you need to include. The course uses the above template, so you can work through the material it to fill in the template as you progress.