Normally the leader of the second largest party in the House of Commons becomes leader of the opposition.
Her Majesty's Opposition is usually the second largest party in the Commons.
Being leader of the opposition means you must act like a prime-minister-in-waiting. You have to keep a close eye on what the real prime minister gets up to, and let the public know what you would do differently.
In the same way that the Prime Minister forms a team called a cabinet, the leader of the opposition forms a shadow cabinet. This acts like a 'government-in-waiting'.