Before you can start your driving lessons you must apply for, and have recieved your provisional driving licence (this is your licence to learn to drive).
You can apply for your provisional car driving licence upto 3 months before your 17th birthday. However, it wont become valid until you turn 17.
You can apply for a provisional driving licence online using the official DVLA website at :
You can start preparing for your theory test straight away using any of the various products available.
I recommened Theory Test Pro, which all my pupils get free access to.
Although you can start your driving lessons once you have your licence at 17, and study for your theory test now, you CANNOT take the driving test until you have PASSED the theory test.