Remember, the light meter is most often indicated by a circle, half circle or something close to a circle in the center of the viewfinder (see above.) This example is for Canon and I don't have a Nikon example but they are very similar. Also take note of what the light meter indicator generally looks like.
Here's what you need to know:
1. The light meter indicator helps you get a proper exposure
To manually set your exposure you need to make sure the tic-mark is on the "0" and to get it there, you adjust your aperture / shutter speed / ISO. The light meter indicator tells you if the exposure looks too bright (+) or too dark (-) and you may have realized by now, this is sometimes subjective. It depends on what you're taking the meter reading off of, or what kind of picture you're trying to take. If you're light painting, the indicator will show it as way too dark (-) etc. Sometimes even though the tic-mark is on "0" the image is still too overexposed.
2. The light meter isn't perfect
Stay calm and remember - the light meter isn't perfect. In fact the light meter easily gets confused. This is why sometimes no matter what you do to get the tic-mark on "0" the exposure is still bleh! Well in digital photography there's a little thing called "12% reflectance of gray." It's too complicated for me and I'm not going to go into specifics here but pretty much, its how the camera sees the world. Which means black & white are a big deal in light meter land...
3. Black absorbs light
Ditto for pretty-much-black and really dark grays. These colors will confuse your light meter: if you take the meter reading off these things... the camera will think you need more light (when you don't) and prompt you to over expose.
4. White reflects light
Bright white snow, white sand beaches, reflective water etc. will confuse your light meter: if you take your meter reading off these things... the camera will think you have TOO much light (when you don't) and prompt you to under expose. Well crap!
5. Find a neutral subject
So black and white objects confuse the light meter. What to do?! Take the meter reading off something neutral in the scene and take the shot. That's about it. Confused? Don't worry, I have pictures.
The image above is pretty much how the sculpture actually looked in person. When I took my meter reading off the black sculpture, the camera meter told me to add more light...
but instead I got an overexposed picture. No problem! I just took the meter reading off the neutral sky behind the sculpture, set my light, then recomposed and took the shot. TADA!
Here's how Ava pup looked when I took her picture. It was really bright and lots of light was reflecting back from the sorta-white wall (and concrete.) When I took the meter reading off the wall, my camera's light meter told me to use less light...
and then the picture looked way underexposed... bleh! Instead I took the meter reading off the blue sky. I set my light (my settings) and recomposed the shot.
Just remember the light meter is a guide, it helps you set a proper exposure. It is often great but sometimes gets confused. If this happens try taking your meter reading off something neutral in the scene, set your light, recompose and take the shot. The more you work with your light meter the easier it will get!