Decide first, which suit you want to lead, and, next, which card of that suit.
• If partner has bid, lead partner’s suit unless you have a good reason not to
• Otherwise lead top card of a sequence, or low from an honour in your longest suit (♠QJ1052, lead ♠Q. ♥Q108, lead ♥8)
• With three small trumps, lead a short suit: a singleton or a doubleton-hoping for a ruff
• Lead trumps if the bidding suggests declarer will cross ruff. You're forcing declarer to play two trumps at once instead of one
• Lead your long suit if you hold four or more trumps. It will force declarer, and you might take control
• Lead suits that have not been bid by the declaring side
If it’s your own suit, lead the
• Top card of a sequence or near sequence of three (two cards in a suit contract) eg ♠KQJ52, lead ♠K. ♠KQ1054, lead♠ K. ♠KQ854, lead ♠K if suit, lead low if no trumps
• Top card of interior sequence, eg ♠KJ1085, lead ♠J
• Fourth best (from the top), if no sequence, eg ♠K875 3, lead ♠5. Use Rule of 11. eg if the ♠5 is led and this is a 4th highest card, subtract 5 from 11 (= 6). This means there are 6 cards higher than the ♠5 in the other hands. It can guide the defence to knowing whether to continue or switch, and helps the defender in third seat know how high to play to trick 1.
• Low from three cards to an honour, eg ♠Q72, lead the ♠2
• Top card of a doubleton, eg ♠106, lead the ♠10
• The Ace is a special situation: don’t lead away from an Ace when defending a suit contract (eg ♠A864). It’s often better to lead another suit. In NT, lead 4th highest (♠A864, lead ♠4)