Some great advice above, IKLland pointed out some of the basics for a great image and others have posted links to some good websites. However, for me there are two important aspects that can't be taught, those are passion and patience. I find that I capture my best images by waiting for the subjects to come to me and that sometimes requires being in one place, well hidden, for a couple of hours maybe. I'll scout a location where I know my target species is likely to occur and set up in a place where the light angle will be right and there's a nice clean background. Yes, you can get some great shots by walking around with a camera but the more you have control over the setting the greater the chances of capturing an image that just wouldn't be possible walking around.
Below is a shot of a couple of drake wood ducks that I took a few days ago. I know they were on this marsh and there would be no way that I could get this shot if I approached them in the open so I found a spot along the edge of the marsh where I knew the sun would be rising behind me. I put camo netting over my camera/lens and myself and laid flat then waited for the ducks to come into range. This technique is not for everyone and requires dedication and patience but sometimes it's the only way to get decent results. The same technique can be used for songbirds by placing an attractive perch over a water drip or some seeds, stay hidden and wait.