Depending on the camera you have you should have a wheel for different settings. Leaving it on Auto will try to figure out whats best however it can sometimes be "wrong" or unable to capture what you have in mind so you'll have to use Manual mode to program it yourself.
Keep the exposure triangle in mind.
ISO is how sensitive to light your sensor is. The higher the ISO the higher the sensitivity to light but in really dark shots it can produce noise so you'll have to find the right balance with the other two
Aperture dictates how much light is let in. It's the "iris" of the camera. A wide aperture will let in more light than a smaller one. A wide aperture will also produce a shallow depth of field and blur out the background while a smaller one will take in more of the background. A shallow DOF with lights or color in the background will produce bokeh
Shutter speed is exactly what it sounds like, how fast it will take the picture, more specifically how long it will open the aperture for
Its all about finding the right balance for each shot. For example, if you're taking a shot at night, you'll want a wide aperture and a slow shutter speed with an ISO thats high enough to take everything in but not so high that it produces noise.
If its broad daylight and your ISO, aperture, and shutter speed time are too high it'll over expose the image and the opposite is also true.
This one >>6429011
required a medium aperture, very low ISO, and very fast shutter speed
was taken in a dark room with an LED flashlight so I used a wide aperture, medium ISO, and slower shutter speed
It should be noted that the longer the picture takes the steadier you'll camera will have to be, so get a tripod.
On the flip side, this one >>6416519
was done with my settings all leaning toward a fast shutter speed
Anyway. Figure out the exposure triangle and check out some youtube videos on cinematography to learn more about mood and framing and the like.