Tutorials and examples can also be very helpful. See http://www.graphviz.org/pdf/dotguide.pdf and http://www.graphviz.org/Gallery.php.

I basically did it by trial and error.
I'm no expert - but try experimentation to achieve something useful for you.

Here is my example organisation chart:

digraph graphname {

Kate [shape=record, label = "{ Kate | Top dog}" style=filled fillcolor =cyan3]
Jason [shape=record, label = "{ Jason | Under dog}" style=filled fillcolor =cyan3]
Percival [shape=record, label = "{ Percival | Hot dog}" style=filled fillcolor =cyan3]
Persephone [shape=record, label = "{ Persephone | Good dog}" style=filled fillcolor =gold]
Bluebeard [shape=record, label = "{ Bluebeard | Scurvey dog}" style=filled fillcolor = darkOliveGreen]

Geoff [shape=record, label = "{ Geoff | Old dog}" style=filled fillcolor =cyan3]

Kate -> Jason
Jason -> Percival
Jason -> Persephone
Jason -> Bluebeard
Percival -> Geoff
Percival -> Mike
Percival -> Jeff
Percival -> Florian

Persephone -> Rajesh
Persephone -> Pierre
Persephone -> Peter
Persephone -> Paul
Persephone -> Mary

Geoff -> Jason [style=dotted]

Bluebeard -> SmallChap
Bluebeard -> BigChap
Bluebeard -> MediumChap
}

Put this in a file called 'example.gv'
Run:
dot -Tpng example.gv > example.png

Then try fiddling about with it.

Have fun