One of the best ways to determine what journal to submit your paper to is to talk to more experienced colleagues. Ask them which journals they think would be interested in your article -- which might be different from which journals are the most important in your field. You can also search your paper's topic in PubMed or another appropriate database. There are several resources that will help you do this more effectively:
Case reports are often an author's first foray into publication; however, not all journals accept them. Here are several resources to help you locate appropriate journals.