Guess from your id you might be in Australia.

I hope it is a proper apple tree from a nursery and not a "wildling" grown from an apple core, because in the latter case you are unlikely to get anything nice from it. Did you taste last season's apples to make sure they are edible?

So assuming you have a proper apple tree, what it needs is a good prune. If you are in the N hemisphere now is the right time, but do it now, not in March. In S hemisphere wait till July/Aug. You want the branches to head outwards and not curve back inwards. You don't want any branches crossing other branches so that they might rub in the wind. And you don't want it too bushy, rather it should be a bit airy. So (1) cut out any diseased or damaged wood, or anything just too old to have vigour, (2) cut out one of each crossing branch to stop them rubbing. (3) If it still needs thinning out, then preferentially cut out the less vigorous wood. (4) On what remains, shorten last year's growth back to a couple of buds, and remove anything that looks really thin and weak.

When pruning anything heavy with a saw, cut from below first and then finish off from on top, otherwise it will snap before you cut through and pull off a strip of bark. When entirely removing a branch, you will observe a little collar where it joins the trunk or larger branch: cut back to this collar trying not to damage the collar, the collar is what will cover the cut. When shortening a branch, cut back to a bud, and snip through at an angle so it almost goes behind the bud. A standard gardening book will show all this. It is no longer advised to paint the cut surface on major prunings.

The shock of this major pruning means you probably won't get much fruit the next season, but it should be rejuvenated in future. In following years you try to maintain the structure, and shorten new wood as before. I have successfully rejuvenated an old apple tree like this.

You will need local advice on what to do to discourage local insects, parrots, etc eating your apples. Should be available on web, in books, etc. Probably a good idea wherever you are to put a grease band on your trunk and replace every few months. In winter this discourages certain insect larvae from climbing the tree, which will then become worms in your apples. In summer, it discourages ants. In Britain, that's all I do, (I'm not aiming for supermarket perfection) but many people give it a tar-oil spray in winter, etc. Likely to be specific other things to do in US, Australia, etc.