ISO. If you're shooting with a film camera then you choose the sensitivity of your film depending on what you expect the conditions to be like. Bright daylight? ISO 100 or 200. Darker conditions and you might choose ISO 400 or 800 film.
The same system applies to our digital cameras, except that you can alter the ISO for each image, and it now represents the sensitivity of the digital sensor in the camera rather than the film. But the principle is the same.
Auto or Manual ISO
Most cameras now offer an auto-ISO option, meaning that you can let the camera control the ISO setting for you. You may also be able to set parameters for it, such as the maximum ISO that you want to allow, and the minimum shutter speed.
It's well worth experimenting with the higher ISO values on your camera. In fairly dark conditions, say indoors in the evening, take a few shots of the same scene at ISO 1600, 3200 and 6400. Take a look at the results on your computer, particularly in the dark areas. You should see increasing 'noise' at the higher ISO values. This should give you an idea of how your camera performs at higher ISO values and the maximum value you might want to accept. Personally, on my current camera, I'm always happy to use ISO 1600, and usually accept 3200 too.
Auto ISO can be great when you want the camera to do most of the work in setting the exposure, which usually means in fairly normal lighting. You can shoot in aperture or shutter priority (depending on whether you most want to control the depth of field or control any motion blur) and let the camera do the rest.
Exposure compensation is a great tool if you don't want to use full manual mode (controlling aperture, shutter speed and ISO manually), but you do want to vary the overall brightness of the image from the mid-gray that the camera thinks is correct. Exposure Compensation is simply telling the camera that you want the image to be darker or lighter and by how much. It's measured in 'Stops of Light' - see below.
Know your camera
It's crucial that you get to know your camera. You might come across functions that you don't think you'll use, but it's still important to know what it can do. Read through the manual. Read through the menu system and experiment. Chances are that you'll be able to customize some of the buttons so keep that in mind as you get to know the functions better.
There are also some very good and thorough camera tutorials by Tony Northrup on YouTube. They don't cover every camera but if they do cover yours or a very similar model then you'll probably find them useful: Camera tutorials on YouTube
'Stops' of Light
A 'stop' of light is a measurement of a change or difference in exposure/brightness. For ISO and shutter speed it's easy to measure. Doubling or halving your ISO setting or shutter speed is a change of one stop. So changing your ISO from 200 to 400, or from 1600 to 800, is a change of one stop. Changing your shutter speed from 1/1000 of a second to 1/500 of a second, or from 1/30 to 1/60, is a change of one stop.
Aperture is a bit more tricky. For reference, here's a range of aperture values with a difference of one stop between them:
f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22
The key advantage of knowing this system is that if you adjust one setting, you know how much to change another to keep the same exposure. So, for example, f/2 at 1/1000s = f/2.8 at 1/500s = f/4 at 1/250s.
Metering is the function of your camera that determines the level of brightness of image. If you're in any automatic mode (auto, program, aperture priority, shutter priority), the camera meters the image and adjusts the settings to create an image at what it measures to be a 'correct' exposure. 'Correct' being that the average brightness is halfway between white and black (actually 18% grey, which are eyes see as being mid-grey).
Where the metering system differs, is in how much of the scene it uses to evaluate the exposure, and how. These are the most common modes:
Matrix or Evaluative Metering - The camera evaluates the whole frame
Center-weighted Metering - The camera evaluate the whole frame but puts more importance on the central area of the frame
Spot Metering - The camera evaluates only a small area, around the point of focus, and ignores the rest of the frame
Choosing a Camera
Everyone has their own bias toward a camera brand or style of camera, depending on that they've used in the past and use currently. More me, that's Fujifilm. I'm not sponsored by them, but I've used their cameras since turning professional.
I generally recommend mirrorless camera anyway. Partly for the size and weight, partly for the benefits of an electronic viewfinder (particularly seeing an accurate preview of how the image will look) but also because I think the design approach makes more sense going forward. Sony and Fujifilm have tended to be at the forefront of mirrorless camera, along with Panasonic and Olympus. I'd tend to opt for Sony or Fujifilm because they use a larger sensor than Panasonic and Olympus.
It's not difficult to find camera reviews online. For written reviews, DPReview and Cameralabs are both excellent and extremely thorough. Gordon Laing from Cameralabs is also on YouTube, or for slightly less technical reviews I really enjoy TheCameraStore.
Another suggestion, particularly in US but probably applicable elsewhere, is to consider buying a used camera. Companies such as B&H, Adorama and KEH stock a large amount of used equipment, provide ratings as to the condition, and often have 'open box' or demo models at a decent discount over the full retail price. They also have good returns policies.
Don't just think about the technical specs, consider how a camera feels in the hand and whether the size means you might have it with you more often. Some cameras just got some hands better, and some are simply more fun to use than others.
Consider the lenses too. Personally I prefer to use prime (non-zoom) lenses. They tend to be physical smaller and provide better quality, often at a lower price than a zoom lens.
Get to know your camera. Carry it around the house. Try to use some of the controls without looking. Get familiar with changing the aperture, shutter speed and ISO.
Take a look for an image that you like or admire online, on Instagram or 500px for example. What do you like about it? What is it that gives you an emotional reaction to it? Is that something you might be able to consider for your own photographs?