Two problems with leading. Which suit and which card? The latter is easier.
This is about your first lead, but much applies later in the play as well
1.Standard leads against any contract
Top of sequence , but care if highest card is 10 or less
Low card from single honour
Middle card (7-10) from ‘poor’ suit
1(i)Against suit contracts: do not under-lead an ace, and try not to under-lead a K, do not lead from a broken suit like AQ. Try not to lead a singleton trump unless opponent’s are expected to hold 10 or more trumps between their two hands. A low card lead from Kxx in trumps very rarely costs!
If partner has bid:
Lead highest card of partner’s suit
Lead a singleton or doubleton in an un-bid suit only if you have reasonable trumps and the clear expectation that partner can gain the lead.
If you have supported partner’s suit: always lead an ace, otherwise as standard leads
(ii)Against no-trumps: you may lead from an honour in an un-bid suit. Lead 4th highest from an un-bid suit containing an honour
(ii) If partner’s suit bid has been overcalled 1NT, and if you hold a singleton in partner‘s suit, lead a more solid suit e.g. QJ10x than the singleton
(iii) Against a slam do not under-lead an ace or K. Lead trumps if necessary. Avoid cashing an ace, unless you expect to win a trick in another suit.
2. Which suit?
You often have to make choices that give away tricks.
Listen to the bidding.
Lead partner’s bid suit unless you have a better suit yourself. Do not lead declarer’s first bid suit unless you have a solid suit (QJ109) and will not be giving declarer 3 tricks. In an uncontested auction, lead an un-bid suit.
If you have to choose a suit, choose dummy’s second bid suit
Against a slam, lead safe if you hold cards over declarer’s suits or attack with a KQ honour sequence in an un-bid suit or a dummy suit.
These are often conflicting conditions, so expect to get the lead right around 60% only