The qualifications required for a career in science vary depending on the sort of job you would like to have. You will usually need a fairly solid science education, so getting good grades at school is a good idea. That said, if you're not the best in your science class, that doesn't necessarily mean you won't be able to be a scientist - you just might need to choose your career direction carefully.
For a research career, if you're not getting near top grades at school, you're going to find it hard to get into a good science university. While being really smart doesn't necessarily mean you will be a brilliant scientist, jobs are competitive and a good degree is important. A PhD is vital if you want to one day run your own lab, but if you just fancy doing the lab work then you will be fine with a science degree alone.
If you think science might be cool, but are unsure about what direction to take, my advice is to consider a general science degree for starters. Don't limit yourself to one career path before you absolutely have to. Don't feel you need to specialize at this early stage in your career as its not the knowledge that you learn at university that will be important, but the way you learn to think. Unlike some university courses, a pure science degree should be useful even if you don't decide to become a scientist. Biochemistry, biology, or chemistry will provide you with a good basis for going into a job in any scientific field. Studying a core science subject will teach you to critically evaluate new ideas and give you skills that will be useful in whatever you choose to do next. Problem solving, analytical thinking, team work and communication skills are highly valued by most employers.
Hopefully you will come away from university knowing a bit more about what path you want to take. There are loads of post-graduate courses you can consider after a degree. An MSc or PhD could allow you to specialize further or change direction in your career. Lots of people combine a science degree with a management qualification, going on to work in a wide variety of jobs afterwards. Others might get into writing for the uni newspaper and choose to follow a career in broadcasting and journalism - look for science communication MSc courses if you like the sound of this. Many people move into teaching, while others join investment banking firms and never set foot in a lab again!
On top of qualifications, experience is great if you want to set yourself apart from all the other would-be scientists. While at uni, you could work in a research lab over the holidays. Many labs will take you on (for no pay!) if you bother to get in touch and ask nicely. If you fancy journalism, then get writing - Student newspapers, web sites, writing competitions....You won't get that job with a top paper or magazine if you haven't demonstrated your passion for writing prior to sending off your application!
So in summary: Get a good core subject science degree, take further post-graduate courses to specialize in your chosen subject, and try to get experience before you apply for that job. Good luck!
