I guess the manual is a help in this case. The thing is, when you are not using spot or distance lights, the light actually has no direction (it spreads in all directions). And I must disagree with the possibility of 'seeing how far the light is coming down' with a spotlight. Experimenting is the best way to find out, but then you should use fairly low levels of render quality and sizes. The numbers you should use, for the millions of possible atmosphere you want to create, are not easy to put in a tutorial, because they depend on the size of the objects and the scene you use. Lighting is one of the hard parts, you can not learn it without doing it. If you Google on 'carrara tutorials lighting' most of the results are training cd's or dvd's. For this kind of basic onderstanding of the whole concept of lighting that might very well be a very good start if you do not have large amounts of time to spend, but have some cash.