Exposure triangle

The Exposure Triangle is simply the term used for the three fundamental elements of exposure: aperture, shutter speed and ISO. The following diagram will help you understand more about this three elements in just 5 minutes.
Exposure triangle cheat sheet (source: Digitalcameraworld)
You will need more exposure in low-light condition for example: at night, in door... and vice versa. The aperture is the most limited element in those three. It depends on your lens, and the wider the aperture, the more expensive the lens is. But a kit lens with aperture from 3.5 to 5.6 is good enough for beginner.
You rarely need a iso of more than 1600
And the shutter speed for hand held sometime depends on your zoom level, but with lens' vibration reduction system, you can reduce your shutter speed few stop which is good for low light condition.

@Lam Hoang