Now, I’ll talk a bit about your camera and exposure. For any given light situation metered, there is an ideal exposure value (0) with it’s corresponding aperture, shutter speed and ISO settings. By default, a camera’s light meter will seek to expose anything being metered to appear as 18% gray, or Zone V in the Zone System [I'll later be posting some info on the Zone System]. To your camera, a perfect exposure is making everything look like 18% gray.
Meaning, that if you point your camera at something primarily dark, or spot meter on something dark, the camera will give you a reading that makes those dark areas appear 18% gray. This will over-exposure your photo, blowing out whites and losing all detail there.
The same is inversely true for metering on something primarily white in a scene. The entire photo will be under-exposed with a lot of detail on whites, but with those white areas appearing as 18% gray. All detail in dark areas is then lost.
Essentially, what is happening is this:
For this reason, 18% gray cards exist. Metering on an 18% gray card will give you the most ideally balanced amount of detail possible that your sensor or film can capture at both the dark and light ends of the spectrum. The exposure you are aiming for is likely to be something like this:
That is the ideal shot. Artistically, you may want to shoot high or low key over-exposing white or under-exposing darks respectively.
In any case, if you don’t have an 18% gray card and you seem to be getting photos that are way too dark or too bright you need to pay attention to the scene you’re shooting. Is it primarlily light or primarily dark? Are you spot metering on something that deviates far from a middle gray? If that is the case, there are faster, easier ways to compensate for it than to pull out an 18% gray card for every shot. Your camera has a EV scale that looks something like this:
You’ll need to do some Exposure Compensation. If your photo is too dark then you need to over-expose from the camera’s “correct” reading. so adding one [+1] or two [+2] stops might help.
If your photo is too bright then you need to under-expose from the camera’s “correct” reading by stopping down one stop [-1] or more.
If you plan on doing post processing, on digital you may want to under-expose the photo a bit [-1/3] and for film you may want to over-expose a bit [+1/3]. If you want to know more on why you should do this refer to my previous post [Film vs Digital Tidbit: On Exposure].