Suggested reading, (four obvious choices, plus some more variant ideas)-
Tom Bingham, The Rule of Law
Glanville Williams, Learning The Law
Nicholas McBride, Letters to a Law Student
The Secret Barrister, The Secret Barrister
John Locke, Second Treatise of Civil Government (the political theory underpinning Constitutional liberty)
Walter Bagehot, The English Constitution (a snapshot from the C19, still of partial relevance today)
Geoffrey Robertson KC, Lawfare (war on free speech)
Charles Dickens, Bleak House (describes what was wrong with the English legal system before the major reforms of the 1870s, contains the best opening paragraph in English literature)
Anthony Trollope, The Eustace Diamonds (for two of the most accurate depictions of a solicitor and a barrister in fiction: see chapters 25 and 28).
Patrick Alley, Very Bad People (the Global fight against corruption and kleptocracy)
Tom Wright and Bradley Hope, Billion Dollar Whale (the biggest fraud in history - spoiler: some of the villains have recently been brought to justice by an international team of lawyers)
Blogs, podcasts, substacks etc by legal commentators such as Joshua Rozenberg, Marcel Berlins, etc.
Have a look at essay competitions, summer schools, mooting events and mock trial events. Don't worry about obtaining work experience, because it's hard to come by for sixth formers, and law schools won't expect you to have any, although if you do get the chance to spend some time in a law firm or a barristers' chambers, do so.
Maybe go to see a hearing at the nearest Crown Court, and the nearest County Court. If in or near London, go to the Old Bailey (crime), the Royal Courts of Justice (judicial review, general civil, civil and criminal appeals), and/or the Rolls Building (business and property), and/or the Supreme Court (the apex court, appeals on big questions of law).