This applies to pome fruits like apples and pears, and stone fruits like peaches, apricots and cherries. Once fruit trees start leafing out, it’s a good time to prune because you can see and remove the branches that became damaged or diseased or didn’t survive winter. You can also remove suckers from the base of the tree, thin out the branches and trim the whole tree to increase the fruit and keep it smaller. I encourage you to use the excellent resources of the Master Gardener program in California to get free advice on pruning your fruit trees. Or ask any experienced gardener. Your tree will be around for many years and you want to treat it well!
If you want to start with some online research about fruit tree pruning, here is an excellent article (with drawings) from Modern Farmer.
Be careful with citrus!
Citrus trees (orange, lemon, lime, grapefruit) appreciate having the suckers removed, being trimmed and shaped from the outside and from the bottom to make a nice, vase-shaped tree. This can be done at any time of the year. Unlike pome and stone fruit trees, which benefit from having branches taken out of the inside of the tree to increase light and air circulation, citrus trees will get burned and damaged from internal pruning. So keep to the outside of the tree!