You can apply for your provisional car driving licence up to three months before your seventeenth birthday (it would not become valid until you were seventeen).
To apply for your licence the simplest way is online https://www.gov.uk/apply-first-provisional-driving-licence
There are other ways to apply https://www.gov.uk/apply-first-provisional-driving-licence#other-ways-to-apply
When you receive your licence you will get two parts – a card part (credit card size) and an additional paper counterpart – you need to keep BOTH parts.
Your instructor will expect to see both parts of your licence on your first lesson so please ensure you have them. They could refuse to take you out if the counterpart of the licence is missing!
You will also be expected to present both parts of your licence at your driving theory test and driving practical test.
A Provisional Licence allows you to drive a car on the road provided you are insured for that car, and are being supervised by someone who is over 21 years old and has held a full licence for a minimum of three years.
You cannot drive on the road without a licence.
You cannot gain a full licence to drive without supervision without first having a provisional licence, and passing the driving theory test and the practical driving test.
You do not pass the driving lessons. You learn in the lessons, you then pass (or fail) the tests.
Therefore, to learn to drive and to take the tests, you need both parts of your provisional licence.