...need to trim is but I dont know how? Ive got some books but they are hard to understand do I trim just below a leaf or just above it and how far can I trim it down the branch? Its quite a young tree about 7 years old.

Normally when you trim "prune" a tree, you want to cut below the leaf... or second node on branch

When you prune a tree especially a young one be sure not to cut alot of foliage of because thats how the tree make food. Then you want to cut at the collar of branches.