Pruning is carried out during the dormant season from leaf fall onwards.
How depends on the size of tree, vigour, etc. and what you want to achieve.
The simplest thing to do is to prune away branches in the centre of the tree to open it up and allow light in to most of the branches. Prune out diseased and crossing branches (which will rub together and compete for light.
Prune out long 'water shoots' if present.