Certified GKH Gardening Expert
The best time of year to prune is in February, once the danger of frost has passed but before any fruit has begun budding/blooming.
First you need to do some cleanup pruning. Prune out what is dead and any weak branches, especially if they are crossing other branches. Once the clean up pruning is done, if you have any fruit left on this tree, you need to thin it out. This article will help with that:
Here is information on pruning apple trees: http://extension.unh.edu/resources/representation/Resource000582_Rep604.pdf