Learning to meter the light in a scene takes time, there are many different lighting scenarios you will come across, and there's a few different techniques you can use to decide on the right exposure. A good way to start would be to change the metering mode on your camera to "Partial" now your camera will only meter light from a 9% circle in the centre of your frame.
So as you find something interesting to photograph, say a landscape scene, you need to decide on a rough composition first, then take a look at what is in your frame, what light sources are present? What are the darkest and brightest parts of the scene you have chosen? What part of the scene you have chosen has average light values? You then have to consider what is most important in the scene, depth of field or capturing motion in sharp detail, this will mostly determine whether to lock down the aperture and adjust the shutter speed to suit, or vice versa.
This is why you will eventually figure out the need for faster and sharper lenses, and the 50mm f1.8 mentioned by Village Idiot above is a good place to start, a cheap lens indeed considering its sharpness and speed.
With time and many photographs, you will improve at seeing light, and being able to "Read" a scene or time of day without even looking through the camera or relying on its light meter. A good metering technique to start with that works with a lot of general photography is to meter with your camera the brightest part of your image, then change your exposure accordingly so that the brightest part of the image is reading at around 2 stops over, this way you retain detail in the brightest parts of the image, while also getting a decent exposure for the rest of the scene. Its how I meter most often, and is a good way to get a good base exposure, then adjust as necessary from there.
All the best with learning to use the 400D, the more time you spend with it, and the more you shoot and analyze you images, the faster you will pick up on how light works with your camera.
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