The light next to your webcam comes on only when it is in use. If the light is on, but you aren't using it, then your computer probably infected with malware that lets someone watch you through your webcam. You should take your webcam offline, engage your computer's firewall to try and block the malware, and scan your computer for the virus.
If you believe that your computer is infected with webcam malware, then you should immediately keep the person controlling the virus from getting any more footage off your webcam. Disconnect the device if you can, and cover webcams that are built into the screen with opaque tape or a sticky note. As low-tech as these steps may seem, they will make it impossible for a hacker to spy on you.
Engage a Firewall
If you haven't done so already, you should turn on your operating system's firewall. Webcam malware has to establish a connection from your computer back to the hacker's computer or server in order for him to actually get the data from your camera. Firewalls can prevent the malware from establishing these connections and keep the hacker from actually getting anything from your computer.
Webcam malware isn't any different than any other virus: if you want to identify and remove malware then you'll need to run an anti-virus scan on your system. If you don't already have security software on your computer, choose from among a variety of free and open-source applications you can download and install. Different scanners perform better at identifying different malware strains. You should consequently try scanning your system with different anti-virus software if your first scan doesn't turn up anything.
The Nuclear Option
If your anti-virus scans fail to come up with anything, or their attempts to remove the malware fails, then you might have to resort to the nuclear option: backing up any important files and re-installing your operating system. While inconvenient, it will certainly remove the malware and remove the threat of someone spying on you through your webcam. However, you could find your computer re-infected if you continue to click suspect links or files from strangers.