Do you at least know which research area(s) your supervisor is interested in? If a topic hasn't been specified then I'd assume that you'll be pursuing a topic in your supervisors' area of interest and expertise. Although I'm sure you'll quickly find direction when you start, I too would be worried at this stage as it all seems awfully vague. Also "studentship" implies funding which is usually secured only after a research plan has been produced. Have you been in any contact with your supervisor this summer? It may be an idea to make contact not only to register the fact that you're looking forward to starting, but it will allow you to mention the fact that you're a little apprehensive as you don't know what to expect when you start, you don't know how your 3 years will be timetabled, etc....