Master the light. There you have it. Simple as that. Without a doubt, if you can get the lighting right, then the quality of your photography output will explode exponentially. There are two main reasons for this - science and art. On the science side photographs were historically produced by exposing individual bits of film to light.
In the modern world, digital cameras have electronic sensors that take the place of film but the concept of exposing an image hasn't really changed. Too much light and the image becomes over exposed. Not enough light and the image becomes underexposed. Just the right amount of light and you'll potentially end up with a lovely picture.
Figuring out this balance is the tricky bit hence why so many people probably shoot in automatic mode. Shooting in auto means that you are trusting the camera to figure out the science (i.e. how much light needs to be allowed to hit the sensor) leaving the photographer free to focus on spotting, framing and capturing a nice shot! Trust me though - photography is so much better in manual mode!
Being able to control the type, volume and quality of light hitting the sensor is the absolute key to taking better photos. I totally get that it can be daunting to make the jump from auto to manual but it's totally worth making the effort. At a minimum, if you're still not ready to make the jump to shooting in manual then it's well worth at least trying to better understand the impact that the quality of light can have on your photos.
For example, shooting in the middle of the day or in low light conditions will never yield great results because you'll likely either have too much or not enough light. Shooting (outdoors) around dawn and dusk will always yield better results because the light will be softer or not as harsh. Avoiding areas of to much contrast is also important too - ideally we want to be shooting with the light coming onto our subject from behind us.
As one advances one can think about choosing a camera that can better cope with contrast - one that has a HDR or High Dynamic Range feature. Many phones these days seem to have built in this feature and the HDR range can also be used as important benchmark value when comparing cameras. When one advances even further one might like to start uses filters - these graduated pieces of glass help to physically control the light entering the lens and can be very effective at helping to balance the light in an outdoor scene - especially when taking landscape photos.
Most phones these days also enable their users to set and adjust the exposure of shots simple by touching on the screen and dragging a slider to make the shot lighter or darker. Typically (especially with iPhones) touching on darker parts of the scene will brighten it while touching on brighter areas of the screen will typically darken the entire shot. Figuring out how to use reflected light and how to catch light being bounced off surfaces will help loads too.
My final tip is to either move around when taking photos of people or objects or else move them until you are happy with the lighting. If you can start making light, shadows and darkness in your environment work for you then you will almost immediately be able to start taking better photos. If you can then add more control within the camera, by switching to manual, and outside the camera by starting to use electric (fixed and strobe / flash) lighting then you'll take you're photography to a whole other level!
All this lighting talk can sound very intimidating when starting out but trust me - it's well worth making the effort and you'll be surprised how quickly you'll pick it up. As always, if you have any specific questions then please do feel welcome to contact me directly here!