Auto-focus requires adequate lighting and a subject with adequate contrast. It may not focus in a very dark room, for example... or a night scene. It also needs contrast -- for exmaple if you point the camera up into a blue, cloudless sky it wont be able to focus on that either because there's nothing of contrast (the sky looks the same overall "blue" whether in focus or out of focus so moving the focus makes no difference and this confuses the camera.)
If, however, you have good lighting (e.g. suppose you're outside on a bright sunny day) then the camera should focus on any normal subject then it should have no difficulty focusing (as long as it isn't a completely plain, featureless subject that offers no contrast.)
The camera actually has TWO different focus systems. If you look through the veiwfinder then the camera uses it's "phase detect" auto-focus system (this is the fastest focus system.) If you put the camera into "live view" mode so that you use the rear LCD to compose your subject then the camera uses it's "contrast detect" auto-focus system.
If neither system works with the lens in auto-focus mode then the most likely cause is a problem with the lens (either the camera is unable to communicate with the lens or the auto-focus motor isn't working, etc.) It's far more often that this is a lens problem and not a camera problem.
If you happen to have another lens, try switching lenses to see if that makes a difference.
Are the camera and lens both new?
Tim Campbell
5D III, 5D IV, 60Da
