EXPOSURE
So, you got to know the AUTO and PRIORITY modes, time to tackle MANUAL! It might seem intimidating at first, but with the tools I give you, and lots of practice, you'll master this mode. Basically, I will talk about APERTURE, SHUTTER SPEED, and ISO (ISO stands for International Standards Organization, this organization sets the standards for camera sensor/film sensitivity).
APERTURE
Changing the aperture values (F-STOP) affects the amount of light hitting the sensor, it also increases or decreases your DOF. The higher the number, less light comes through, but more things come in to focus (think: squinting) due to a more ample DOF. The lower the number, the more light comes through, but your DOF is thinner and it is more possible to miss focus. If you are shooting landscapes or if it's a bright sunny day, I recommend using a higher value such as F16. If portraits are more your thing, or you are shooting in a dark environment, use a lower value such as F1.8.
SHUTTER SPEED
Changing shutter speed values affects how long the light hits your sensor. Shutter speeds can also help with your composition by freezing or showing movement. For example, adjusting this value can freeze the movement of flowing water with a high shutter speed (ie: 1/1000 of a second) or show the movement of a jogger by adding some motion blur with a low value (ie: 1/20 of a second).
ISO
Changing the ISO will do two main things for you. First, it will increase or decrease the sensitivity of the pixels on the sensor. The lower the number, the pixels will be less sensitive and need more light to register an image. The higher the number, the more sensitive the pixels, and less light is required for a capture. ISO sounds great, right? Time to go out and shoot in the dark, right? Well, not really, there is a catch: NOISE/GRAIN. Noise (or grain) is a type of texture in photographs that is the result of increasing ISO sensitivity, the higher the ISO, the more noise you'll get. For sharper images, photographers will usually use the lowest ISO value possible, although some photographers use noise as a compositional tool (ie: to make black and white photographs appear more 'gritty').
There you go, the holy trinity of photography (aperture, speed, ISO)! These have been photographic tools since the beginning of the art, and I doubt that it will change. I suggest you play around with these values. Before you shoot, think: do you want to freeze the people or show them move? How about blurring the background or seeing until the horizon? Maybe you want to capture a gritty street scene at night, or a nice sharp sunlit image? There are exposure values for all that and in between, be creative.
I have included a popular cheat sheet that's been circulating around the internet, I suggest saving it to your computer, tablet, or phone
OK! That about does it for basics, we covered MODES, and now EXPOSURE... get out there, shoot, and keep practicing! It's the best way to get good.