1] Start with healthy stock.
Try to buy fruit trees grown on EMLA or virus free rootstocks, and froma nursery specialising in fruit that achieves good standards of husbandry. It's far easier keeping an already-healthy tree in good health, than it is trying to get clean a tree that had bugs or diseases to srart with.
2] Grow newer disease-resistant varieties
Choose varieties that are described as having good disease resistance. These tend to be much easier to grow well and more forgiving if you are a first-time fruit grower. A lot of the older varieties have some wonderful qualities but they can also be more prone disease[there are exceptions] Wait until you have a little more experience before trying these.
3] Avoid high nitrogen fertilizers
Fertilizers rich in nitrogen encourage a lot of lush new growth. This is a good thing, yes? NO! This is just the type of growth diseases and creepy crawlies love. Aim for a sustained and moderate rate of growth. Use balanced feeds like bonemeal, growmore or seaweed maxicrop.
4] Don't be afraid to prune.
Especially with the aim of keeping a fairly open crown. You don't want a congested leafy centre to the tree; this cuts out air corculation and light and encourages sooty moulds and mildew.
5]Feed the birds!
Encouraging a good population of bird-life to stay in your garden pays rich dividends because they will in turn rid your trees of caterpillars and aphids. It's true and it works - I speak from experience!