The first thing a photographer should do is get to know all the features of the camera. Even if you don't use all the features (most people won't) you should at least know what they are so if you need to use them you know it's there.

Program Mode
Program Mode is the fully automatic mode that makes the camera select shutter speed and aperture.

Aperture Priority Mode
Aperture Priority Mode lets you select the aperture and the camera selects the shutter speed hat according to the in camera metering system will correctly expose the picture. This is often used if you want to control depth of field.

Shutter Speed Priority Mode
Shutter Speed Priority Mode lets you select the shutter speed and the camera selects the aperture that according to the in camera metering system will correctly expose the picture. This is often used to control motion. Use a fast shutter speed to stop motion and a slow one to blur motion.

Manual Mode
Manual Mode leaves all the decision making up to you. You set both the aperture and shutter speed.