You can use a program called Webeecam, I am assuming you are using a USB Webcam, hence you would need a OTG Adapter ( Plugs in to your phones microusb port and has a FEMALE USB end to plug the camera into.
Current supported 3rd party cameras
Currently supported Third Party Cameras:
MICROSOFT LIFECAM CINEMAMICROSOFT LIFECAM HD - 3000MICROSOFT LIFECAM STUDIO [HD-1425]LOGITECH HD PRO WEBCAM HD - C920LOGITECH WEBCAM HD - C525LOGITECH WEBCAM HD - C615LOGITECH WEBCAM HD - C310LOGITECH WEBCAM PRO - 9000LOGITECH B910 HDCreative LIVE! CAM SYNC HDEtron Technology USB 2.0 Camera