What if I am not yet 17?

There is no reason that you should not start your preparation early. You can start by studying the Highway Code and reviewing the requirements for the Theory and Practical Tests.
 

You can apply for your provisional licence up to 8 weeks before your 17th birthday.  You can pick up an application form at your local post office or you can also apply online at the www.direct.gov.uk website. There is more information here on turning 17.