You may be able to run up and down the court in a flash, but how well do you move side to side? Your lateral speed is one of the most important factors to become a better defender in basketball. To improve, start by activating your muscles with a proper dynamic warm-up. There's a lot of acceleration and deceleration needed so it's important to loosen your hips and groin first with movements like lateral squats and drop lunges.
Next, master your skills with a mini band. If you've never used a band before, pick one up—it's cheap and it travels. Use it for lateral band walks. Watch this video for a demonstration:
In a defensive position, you want to stay low (you can always come up), so maintain a low stance during your lateral band walks. It's a good habit and it'll also help build endurance in your leg muscles. Loop the band over your knees, get in a quarter- to half-squat position, with your toes straight ahead and the weight over the middle of your feet, and then sidestep. Pick a point on the horizon and make sure your eyes aren't moving up and down. It's important for posture, and it's another good habit to help you keep the defender in front of you. Try 3 sets of 6-12 reps in each direction.