If you have your camera in green auto mode or aperture priority mode, your camera will make a decision about the exposure. Most times it will make a good decision however there are times when the light is tricky and you will want to decide for yourself.
First of all you need to choose a metering mode. This is the number of sensors the camera evaluates to come to a decision. In most DSLR cameras these modes are referred to as Multi-segment, Center-Weighted, and Spot.
Multi segment evaluates the entire scene, Center just the middle and spot just the point you aim at. In good light multi segment is OK most of the time. In some circumstances such as photographing surfers center is best so you get the face properly exposed. But when its dark and the light is highly variable like in a rainforest, you want spot to ensure you properly expose what you are trying to shoot. Rainforest light is very tricky and patches of sunlight breaking through the canopy will render multi segment unusable. Just saying.
Having made your metering mode decision and with your camera in manual mode, set the aperture and use the wheel to dial in exposure. On Sony cams, you compose, hold the shutter half way down, move the wheel until the exposure compensation meter is set to zero. That’s the one with plus and minus numbers and a zero in the middle. And that’s it. Perfect exposure.
The image below is a good example of a situation where highly variable light which would fool a cameras automated metering can be controlled easily using the manual technique described. This scene is evenly exposed and you can even see right into the pool at the bottom of the gully.
This technique may vary with other brands but check it out. On some cameras with a live view, you can see directly on the rear screen the effect of exposure settings. Its a useful technique to know and you’ll wonder why you haven’t done this before.