Defense is the hardest part of the game. You'll end up on defense about half the time, so honing your defensive skills is a prerequisite to becoming a better bridge player. 

It's essential to have solid agreements with your partner about your signals and leads. What sort of signal applies in a given situation? Which card do you lead from different holdings? 

But leading the correct card and giving good signals are not what make a great defender. You need to peer into declarer's mind, figure out what they're trying to do, and counter their attack. You need to visualize the two unseen hands. You need to start with a sound and well-thought-out strategy for the defense. And just as importantly be nimble enough to change that strategy midstream.

One of the most basic and most profound decisions you have to make on defense is whether to defend actively or passively: do you need to take or set up your winners, or do you sit back and let declarer struggle through the hand themselves? 

Adam Parrish covers these topics in more in this comprehensive look at the modern defensive thought process.