Defending a hand in bridge is the same as declarer play in the sense that you're simply trying to use your good cards effectively to win as many tricks tricks as possible, establish your long suits and score extra trick with your trumps.
There is a big difference between declarer play and defence, however, and that is that if you are declarer you can see all your resources, the 26 cards you're going to play. When you are defending you can only see your own cards and dummy so successful defence requires good communication with your partner. 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.
Making a plan
Lead ( and keep leading ) your long suit.
Establishing a long suit is the most common way to develop extra tricks for both declarer and defence.
On the following hand, the best lead is a heart, not the ♠A.
Lead your long suit!
By leading a heart, you will knock out declarer's ♥A. If you can regain the lead later with a spade you'll be able to enjoy 4 extra heart tricks. If you take your ♠A before establishing hearts you'll have no way to regain the lead.
Returning partner's suit
The defence need to work together so if your partner has led a suit then it's often going to be best to keep leading that same suit should either defender regain the lead.
This time West is declarer in a contract of 3NT and North, your partner, leads the ♥K. If you are able to win a trick, maybe with the ♠A, you'll know to return partner's suit rather than attempt to set up your own suit.
Return partner's suit!
If your partner is on lead You're simply aiming to set up your tricks and the opening lead is an indication of where your tricks might come from. You often will return partner's suit but understanding why it's often right means you'll know when it's wrong, too.
Try for ruffs
If you can leave yourself with a void in a suit then you may be able to ruff before declarer has drawn trumps.
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.
Leading a singleton against a suit contract.
Sometimes you may be able to foil declarer's plan to win extra trump tricks by ruffing by playing trumps yourself.
On the following hand, North leads a diamond against West's 4♥ contract. You win the first round of diamonds and immediately switch to a heart.
Play trumps to prevent ruffs in the dummy
If you can remove the two trumps from dummy you might be able to win three or even four diamond tricks.
If trumps are breaking badly for declarer then it can sometimes be worthwhile forcing declarer to use trumps leaving you in control.
You lead the ♠A then the ♠K against 4♥ contract. East ruffs the second round and plays the ♥K. What now?
Force declarer to ruff
Winning the ♥A then playing another spade will force declarer to ruff and you'll end up with more trumps than East!.
Sometimes you're defending a hand and you can see that declarer's finesses are going to fail and that nothing is breaking well. Perhaps your opponents appear relucant to bid game. Make a lead that will give nothing away.
East and West have crawled into game and it looks like no suit is breaking well. A club lead gives nothing away.
Time to go passive!
Your heart and diamond honours are probably sitting over the top of East's honours. Wait till East plays an honour and then simply play a higher one.
Although the defenders can't see their partner's cards, the opening lead can give a lot of information. The lead of an honour generally shows the honour below. The lead of a low card shows an honour.
When a defender is not contributing to the trick, either simply following suit or discarding, the choice of which low card to play can be used as a signal to help partner decide what to do. There are different signalling methods and each one is used in a different situation.
Rule of 11
Leading the fourth highest card in a suit can help partner determine the location of the missing honours with the help of the rule of 11.