The apple tree looks like it is doing well in the environment, now the question is how would you like the mature form to be? Knowing what form, height and spread you want will dictate the type of training/pruning you do now. It is all about training at the is time.
I am going to provide you with a few links to information sheets that give you step by step instructions and good visuals.
It is important:
Do not leave stubs. This is difficult for the tree to compartmentalize around.
Do not seal cuts with anything. Allow the tree to take care of itself.
Cut above a bud pointing the direction you want the future branch to grow.
Avoid training branches to cross over each other.
Don't be afraid to be a little aggressive as you are training, just make sure you step back and look frequently.
Sanitize your pruners after each cut, to be safe
After a couple of cuts stand back and look at the tree. Make sure the tree is still looking the way you envisioned.
You have two main branches and one looks as thou it is already creating an open center. Depending on your goal, if it is high fruit production you could prune for that way. If you want production and a more symmetrical tree, I would suggest not going too extreme on the open center. When fruit trees are completely bare of leaves in the center, branches tend to sunburn causing weakness in the tree.
At this time you are not following the 20% rule, but I would suggest doing training and pruning out water sprouts and more training in the fall before the leaves drop.
Please let me know if you have further questions.
You can also text photos and questions to 775-399-8522