Working through past papers is one of the most effective ways to revise for your exams.
You can get a feel for the type of questions you're likely to face and the level of knowledge expected. You can also work through whole papers in self-imposed exam conditions, so you get a good feel for how much you can achieve in the time available.
All of the UK exam boards make their past papers available for download on their official sites. Below are the direct links to the pages where you will find them. Just click on the link for the board that runs the exam you're revising for and pick the relevant subject.
The Cambridge International Examinations page isn't set up quite as simply as those of some other exam boards. This link will take you to the top level. From there, click on your level, click on the Subjects tab and select your subject. Scroll down the page to find the past papers.