Read books about the law and the Constitution, for example The Rule of Law by Tom Bingham, The Secret Barrister, the British Constitution by Bagehot, Jonathan Sumption's essays, and John Locke's Second Treatise of Civil Government.
Read the Economist and the FT Weekend.
Maybe enter essay competitions with legal or current affairs topics, and perhaps also debating or mooting events.
See if you can obtain work experience at a law firm or in a set of barristers' chambers. Go to criminal and civil courts to watch cases.
NB, don't worry if you can't obtain work experience - there are not many work experience spaces for pre-university students.
Good luck!