You cannot start to learn to drive on the public road before you are 17, although you can start driving off road before then. In fact some driving schools offer off road lessons for under -17's. You can start learning for the theory test before your 17th birthday, but you cannot take the theory test till you reach 17. You can apply for your provisional licence in advance of your 17th birthday.
The theory driving test consists of two parts - a multiple choice question part and a hazard perception part. You have to pass both parts at the same time. If not you have to take both parts again. You can take your theory test before you start your driving lessons, but you may not apply for your practical test till after you've passed your theory.
It's often best to choose a driving instructor that is recommended by friends. Do not automatically go for the driving school that is cheapest or has the biggest name - they may not give the best tuition. A list of driving schools is available at Wanadrive.