That's a huge question. I'm going to assume you're a beginner, because of the lack of jargon in the question -- apologies if I'm wrong about that...
Anyway: there are plenty of resources to learn about exposure -- I'd recommend the book "Understanding Exposure", but I'm sure you'll get plenty of suggestions to choose from.
[EDIT: added the next two paras because I felt guilty about just saying "read this book!" and nothing else]
For a quick fix, you could try using exposure compensation. This lets you tell the camera to make the picture darker or lighter; it's normally indicated by a scale that goes from EV -2 (darker) to EV +2 (lighter). Your camera manual will tell you how to use this; if you're in automatic mode, you'll probably have to switch to program auto first.
For a real fix, you have to understand why your camera is making the scene too bright or too dark. If you're in automatic mode, that will be down to metering. Most cameras will give you a choice between measuring the amount of light at a single point ("spot metering"), or averaging over the whole frame, or some more complicated option. Have a look at the metering options in your camera's manual, and experiment with them -- try taking a few pictures of a scene that contains both very bright and very dark areas.
The problem of colours not being correct might be to do with the white balance on your camera (if there's an overall colour cast - say, everything looks blueish, or yellowish), but that's just a guess.