Some info here http://www.gardenersworld.com/forum/fruit-and-veg/apple-tree-problem/92639.html which I hope is helpful Sara. Good luck
If you look in the Insecticide selling area of any reasonable size Garden Centre, you will (usually) find Winter Oil Wash. It is a horticultural soap made from plants and reasonably organic. The instructions on the bottle are very specififc.
As for keeping the dreaded Wooly aphids under control, then it is best make up a solution and scrub the tree with it.
The aphids themselves have young which overwinter in cracks in the bark, so again spraying will help control them. They are not woolly like the adults so insecticides will work if it reaches their hiding places.