You cannot take your theory test until your provisional licence becomes valid. You can apply for your licence 3 months ahead of your 17th birthday but you still have to wait until that date to take the theory test. So the earliest you can take the test would be 17.
The theory test consists of two parts. First there are 50 multiple choice questions followed by the Hazard Perception test. Multiple choice passmark is 43 out of 50.
The Hazard Perception consists of 14 video clips of various driving situations. On each clip there are a number of potential hazard with one that develops into a real hazard. You must spot the developing hazard, and click the mouse. You score between 0 and 5 points depending on how early you saw the developing hazard. On one clip there are two developing hazards. The pass mark is 44 out of 75.
Do try out the Hazard Perception at home first, there are some cd's available with loads of clips to try out. Don't just think you can click haphazardly on the test. The system senses this and concludes you are cheating and scores you zero points for the clip!