All cameras are able to take infrared pictures, but all cameras have an IR light Blocking filter in front of the camera’s sensor. Some IR Blocking filters are extremely strong and block out all IR light, while others aren’t very strong, and let some IR light pass through.
If you have a camera with a weak IR Blocking filter, you can buy an external IR filter that screws on the front of your camera lens. This will block all visible light and only let the IR light pass through. If your camera has an extremely strong IR Blocking filter inside, I would recommend removing the internal IR Blocking filter completely and replacing it with a filter that blocks visible light and only lets IR light though.
Now, you might be asking “But Evan, how do I know how strong the IR Blocking filter is inside my camera?” Well, in order to figure out how strong it is, find a remote control that goes to your TV or something similar, then take a 1 second exposure. During that exposure, click the power button on your remote, pointing it towards the camera lens. When you look at the picture on the LCD preview screen, you should hopefully see a light coming from the front of the remote. That is near infrared light! Sweet.
If the TV remote light looks bright, your camera is well suited for infrared photography as-is. If the TV remote light looks very dim, that means the IR Blocking filter is pretty strong, which means you can still do IR photography but the exposure time will have to be longer when using an external IR filter. To give some comparison, a D50 camera has a weak IR Blocking filter and the exposure time is about 1 second in daylight when using an external IR filter. A D300 has an IR Blocking filter that is stronger, so the exposure time will be about 20-30 seconds in daylight.
Try doing this test with different lenses; some are better for IR photography than others.