Hetty, In the spring would be best now. Where live, you can sometimes prune twice in a year.
If you prune in the Spring when you see it producing new growth is the best time. The shape is a preference of each of us. I really like the "Mushroom or Umbrella" shape.
You may need to prune it down more the first time in doing so, to get the initial shape started.
I find when I prune mine they throw out a nice set of flowers.
Your tree has a nice branching from lower down to give a nice shape and with it's show of flowers would look quite beautiful.
Rick
