Although a social work degree will provide you with much of the theoretical and practical professional knowledge you will need to succeed as a social worker, studying another subject will make you a more marketable, more well-rounded professional. Here are some minors that will complement your social work major nicely:
Sociology is the study of the development, structure and functioning of human society and social groups. It is a far-reaching academic discipline, addressing diverse subject matter that ranges from religion and the family to crime and penology. Interactions between individuals and larger groups are the bread and butter of social workers' practice. A sociology minor will provide you with a solid theoretical understanding of many of the phenomena you will encounter in your professional life, and equip you with tools that will help you better serve your clients.
If you agree that the definition of politics is who gets what, when and how, then you cannot deny the importance of studying political science. Understanding how the government works at the federal, state and local levels, as well as how each level's resources are allocated, is vitally important for every social work professional. Government agencies employ a large portion of the nation's social workers, and even those who are not employed in the public sector are subject to rules and regulations at each of the three levels mentioned, so as a social worker you will never be completely free of the political sphere.
Humans' motivations for their actions and their responses to particular stimuli is of great importance to many aspects of the social work profession. Recognizing that an understanding of and an ability to analyze individuals' psychological processes is central to your success as a social worker and in any other professional capacity will allow you to be more in tune with your surroundings, professional and otherwise.
Being fluent in a second language will do wonders for your career. You will be far more marketable than your competitors who speak, read and understand only a single language. More importantly, though, you will be able to communicate with and serve a larger portion of the nation's disenfranchised populace. The Latino population is underserved in many ways, and mastery of Central American and South American Spanish will greatly enhance your ability to serve the country's—and, for that matter, the continent's—at-risk individuals, families and communities. Knowing Spanish will also provide you with an advantage if you are interested in pursuing a position in, for example, the Peace Corps or any other service-oriented government or nonprofit organization.
Any one of these minors or some combination thereof will complement a social work degree quite well. You should take your academic and professional interests into account before choosing to declare one or more of these minors, and should likely take entry-level classes in any minor you are considering unless you know you possess an undying passion for that subject. If that is the case and you are still able to do so, you should consider picking that subject up as a second major.