Most plants can be lightly pruned during their growing season. If you want a denser shrub you might trim it once a month to encourage more branching. Other plants will re-bloom if you trim off the old flowers. This kind of pruning just depends on what you want your plants to look like.
Fruit trees and roses should only be pruned in the early spring while they are still dormant. A good rule of thumb to follow when trimming your trees and shrubs is to never remove more than roughly 30% of foliage during one season.
New fruit trees benefit from having their first crop of fruit removed. It takes a lot of energy to make fruit and it is better for that early effort to be put into growing roots and leaves. The shape of the tree you want is all up to you! There are several different shapes and styles for pruning, and it depends on what you want your tree to do for you. There is a great guide from UC Davis for pruning fruit trees that can be found here.