Mastering working with ambient light isn't that complicated. It actually starts with on simple decision. For every place where you want spend time to create beautiful photographs, make sure to decide what the main light source is. It could be the setting sun, a large window or a strong light bulb or neon. That main light source needs to be the one casting the strongest light on your subject.
Then, look around and try to evaluate where the light from the main source goes., how far into the room does the window light reach, what it hits? You need to be aware of how this light acts to be able to use it.
Instead of finding an object or a person to photograph, start by looking for an awesome light. Then, find whatever is in that light that you want to photograph, or hang around for a while for someone to step into your lit stage.
When shooting people or portraits, the main light is usually the one that should light up the face. As long as you have a good facial light, you could even let light of different temperatures (and therefor colors) hit your model. Because don't misunderstand me! I am not telling you to only stick to ONE source of light. That will make your photos flat and probably not very interesting. But it is important to make a conscious decision so that the most prominent light is from your main source, before adding a second or third one.
The next time you sit in a café waiting for a friend, spend some time checking out your fellow coffee drinkers around. How does the light from the window play on their faces? What kind of lines does the back-light create? Where do you find the softest light in the room?
The best way to learn how to find the best light is to start looking for it. It sounds silly, but most of the times we forget to look around.