Fruit trees can be tricky, if you over prune you may have to skip a year of fruit in what is known as biennial bearing. This doesn't mean that the tree will become biennial perinately, it just means that you will be missing out on a year of growth.
Typically it's best to prune apple trees in the winter when they're dormant, unless you are just removing broken or diseased branches.
Pruning when dormant generally results in vigorous new growth come spring, but the sap isn't running so you can take larger branches. Pruning during the rest of the year taking large branches can result in a risk for the tree. This isn't a given, but it shouldb e avoided..
You may have been told to prune in winter for structure and this is good solid advice. During th rest of the year you should only prune for size control and sickness, though a well maintained tree will tend to grow in the right direction for a number of seasons before any major work needs to be invested.
Size control is a simple task. You need to decide how tall you want the tree to be. If you only want your tree to be three yards tall you will cut off all new growth above that level. This results in a more uniform thickness and shape for the branches under that mark.
One thing that you may need to do with a well maintained tree is to focus on the fruit control. When they are caredfor they will tend to produce more fruit.
That means you may need to prune even if you have the right shape. Summer pruning can help thin the fruit load, keep the tree to a specific height, and thin out water shoots.
You have to be careful you don't get rid of next years fruiting wood though.
Trees, while not like us, can be infected with a wide variety of disease. If you have questions you can ask the extension office or other organizations in your area for advice.
Generally they like a photo of the whole tree from far away, one of the whole tree closer up, one of an infected branch, then a photo of an infected leaf or blossom.
Then put a magnifying glass up to an infected leaf and snap a shot of the front of the leaf then another of the back so that they can see the details.
Email all the photos to your extension office along with the variety of apple that you're growing and any other symptoms the photos don't show, and they'll tell you exactly what to do to make your tree fruit again!