Not knowing where to start in deciding what career you want is very common and normal. There are a number of starting points and these will be different for different people.Choosing a career is not a once off process. One develops ideas in a series of steps. Careers are constructed over time. This helpful movie about approaching your application is also useful for an approach to career planning and what to expect at University.
Your degree is only one part of what makes you employable and there are many more options open to you than the ones that may be obvious from your subjects.
One way of doing a reality check of careers you are interested in in by conducting informational interviews. This involves meeting with someone in a job that interests you and asking questions about how they got their current position and what it entails. Not only does this give you insight, it also gives you exposure to unadvertised opportunities and will expand your professional network. Find more information on going about this and what to ask here.