Obviously, to use the case study, or the case method, you have to have a case to use. Here's a quick overview of the kind of information you need to provide to the learners.
The case should have a central issue, or the main problem to be addressed, although that doesn't mean that there can't be other related problems (which often reflects reality.
The case must describe the situation, or the events, circumstances, actions, characters and dialogue out of which the problem or issue emerges.
The case must also provide enough background to allow for intelligent decision-making when addressing the case problems. If it's a business case, for example, it might include industry, company size, location or any other details that might be important and relevant.
When designing a case study, it's good to test out your case on people to determine if they feel there is sufficient information and detail to actually attack the problem. Often your perspective as a case designer will be different than learners' perspectives. The case may also contain any documentation, or exhibits to support understanding of the situation. For example, again, in a business case, that might include balance sheets, inventory figures, annual reports. Or, if a medical case, documentation might include a medical patient chart.
In short there should be enough information to make the case realistic to the learners, and enough information so that learners can try to solve the issue without requiring further information.