The opening lead is possibly the most informative card for the defence because it indicates what the leader holds in that suit. Just as importantly, though, the lead often suggests a plan for defending the entire hand.
Your main options are leading from length, leading a short suit, leading partner's suit or leading a trump. The opening leader can also 'show' partner cards by sticking to some well known leading agreements.
Leading your long suit, especially against a notrump contract, is a common way of establishing extra tricks.
When you have a sequence of touching honours leading the top of that sequence let's partner know about your holding in the suit. The lead of an honour shows the honour below but denies the honour above.
On the following hand South leads the ♠Q. North now knows that South has the ♠J but not the ♠K
The next two hands look similar but the opening lead of top of a sequence gives South the right information to defeat the contract each time.
North leads the ♠K against West's 3NT contract. Upon winning the ♦A, South returns a spade, knowing that North has the top spades.
South's hand and dummy are the same on this next hand but the opening lead is different.
This time North leads the ♠Q against West's 3NT contract. If South returns a spade, declarer will win that trick and play all the diamonds. South's only chance to defeat the contract is to switch to a heart and hope North has the ♥A.
The lead of a low cards promises an honour so your partner will know that's the suit you would like to establish.
When leading a low card from a long suit it's normal to lead your fourth highest. Doing so will help partner know the length of your suit.
In this first example, North leads the ♥2 against West's 3NT contract. North has no cards lower than the 2 so he must have started with exactly four hearts.
North started with four hearts!
This time, North leads the ♥6 against West's 3NT contract. You can't see the ♥4 yet so you can't be sure about North's heart length. Once you see the ♥4 you'll know whether North started with four or five hearts.
Who has the ♥4?
South's longest suit is spades but the suit is weak so he leads the ♠8.
Some players like to lead second highest of a weak suit and others still choose the fourth highest even with no honours in the suit. You can talk about it with your partner and make your own decisions, there are pros and cons to each approach.
What does leading away from an ace mean? If you have a suit headed by the ace and you choose to lead a low card from that suit you're said to be 'leading away from an ace' or 'underleading an ace'. That's ok against a notrump contract but not good against a suit contract.
The contract is 3NT by East and South leads the ♠3. West's ♠K makes but as soon as the defence regain the lead they can take four spade tricks.
On the same hand, imagine the contract is 5♦ by East. If South leads a small spade West's ♠K will make and declarer will be able to ruff any further spade leads. That's bad for the defence. Don't lead away from an ace in a suit contract.
Against a suit contract you also have the option of leading a short suit to try and score a ruff.
Here you're on lead against 6♠. You could try leading your ♣4. Partner might be able to win the trick with the ♣A and return the suit for you to ruff. Partner might even win a trump trick and be able to give you a ruff before declarer is able to take away all your trumps.
This time no lead looks particularly attractive but if there is only one unbid suit that can be a good option.
Lead the ♠8, top of a doubleton, and partner will know you don't have an honour in the suit.
If partner has bid, it's often a good idea to lead that suit.
You might have chosen to lead a club but partner bid spades so keep partner happy by leading the ♠9
An opening lead of a trump can sometimes be an effective way of preventing declarer taking ruffs in dummy. One clue to when it might be the right thing to do is when you know dummy's side suit isn't breaking well.
Your double was takeout but partner passed so he must have very good trumps. A club lead will spell doom for declarer and happiness for your side.
When you really don't know what to lead you should eat a peanut butter sandwich. That way if your lead doesn't work out well at least you get to eat a peanut butter sandwich.