In Program mode the camera makes those decisions for you. In Aperture Priority, Shutter Priority and Manual, those settings are under your control. In Shutter Priority, you select a shutter speed and the camera adjusts the aperture based on the light meter. In Aperture Priority, you select the aperture and the camera adjusts the shutter speed based on the light meter. Note that you will want to check the settings the camera selects and perhaps adjust to get the settings you need. The ISO is the third side of the triangle and helps determine the exposure settings as well. The higher the ISO, the more sensitive the sensor is to light, but the higher ISOs can add grain to an image.
Be sure to keep your shutter speed high enough to avoid camera shake. Generally that means a shutter speed faster than the size of the zoom lens or prime lens - 1/60 for an 18-55 or 1/300 as minimum examples. Some folks can hold the camera steady at slower speeds but that is a general reference point until you have the chance to test. I used to be able to hold much steadier than I can now being older. A tripod or monopod for a longer lens or slower exposure can help quite a bit for those situations...