There are several methods of pruning (well, styling), all of which are used to give the tree a different "feel". However, in your circumstance, I wouldn't worry so much about these pruning methods as about the basics. First off, it is important to understand that most trees only produce new branches from buds growing from the leafs. So when you prune, it is important to get it right next to a leaf. Second, if you want a lot of secondary branching on the branch that you're pruning, you may want to defoliate it. It is a good idea to prune down to a bud that is pointing in the same direction you want the new branch to grow.
As for the style, that is up to you. Your tree looks like it could be trained as a broom or an informal upright; your choice.
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