I'm assuming its a small, young tree that you have and I'm also thinking that a possible cause might be due to a lack of other apple trees in your location for cross pollination. if that is the case, then it may be worth planting more.
Prune the tree when it is inactive/dormant, during November to early march at the latest.
get to know the fruiting buds from the shoot buds, fruiting buds are the short stubby ones.
I think it will also increase your chances of a good yield by growing plants which attract bees.
Check on the RHS site for some good advice.