At the moment, it seems you can't, but the foundation are apparently planning to sell cameras you can plug in in the future (or at least they were last year):
Usually the camera requires two interfaces, the CSI 2 or CCP 2 port for the image data coming from the camera, and I2C to actually tell the camera what to do.
Next year the foundation will be selling a camera that you should be able to plug straight in - for the moment the CSI port is fairly unusable without an already supported camera - they need drivers on the GPU which cannot be written 'at home'.
There also the question of getting the camera data in to Arm Linux land from the GPU.
All this stuff will be sorted out when the proposed camera comes next year.
In the meantime you could use a USB camera.