So if you want to be fitter, what should you do? The best solution would probably be to join a gym or a keep-fit class.
By signing up to a class, you gain expertise, benefit from facilities and, perhaps most importantly, develop a routine that is essential if you are to maintain a fitness programme for more than a couple of weeks. You'll be shown what to do, how to do it and someone will be on hand to make sure you're not over- (or under-) exerting yourself.
But if you decide to take charge yourself there are various points you need to bear in mind.
Whatever you do, warm up first and be careful not to overstretch yourself too quickly or you risk causing real damage. Plus, of course, you are less likely to continue exercising, defeating the whole point.
You should do resistance training such as weights to improve your muscles and strengthen your bones. You don't need to use weights, you can use your own body weight, for example by doing press-ups.