How can I improve my chances?
It's great that you have already decided that you'd like to come to Cambridge!
The most important way to help your chances of getting an offer from Cambridge is by working hard in your GCSEs and A-Levels, IB or Highers to get the best qualifications possible. Cambridge sets high standards for prospective students, and the harder you work, the more likely you are to achieve them. This doesn't mean that you have to spend all your time working, though. Try to think about what it is you enjoy, and look around the topic outside of the classroom. A genuine enthusiasm is much more important than an inside out knowledge of your syllabus!
It's always a good idea to try to get some relevant work experience in the subject you are interested in. For example, if you want to do Medicine, it would be useful to have done some work in a hospital. Not only will this show you whether the subject really is for you, it will give you something to talk about when you apply. Similarly, for subjects like Law or Engineering, work experience is a useful way to develop your skills before you get to University.
It might be that you can't think of any relevant work experience, but there will be ways that you can get interested in your subject. You might be thinking about studying History, and take trips to the local museums or heritage sites in your area. You might be interested in Politics, and choose to follow the American Presidential race and elections. Explore different topics, see what you enjoy and what's on offer: your opinions are bound to change, and it's good to keep an open mind.
Extra curricular activities aren't going to be a substitute for academic ability when it comes to applying, but people like to see that you can balance your time. The student motto here is very much "work hard, play hard", and make sure that you take time to do what you enjoy, hang out with friends and relax, too!