My key does not insert or withdraw easily or it has difficulty turning.

First spray the lock with light penetrating oil (Tri-Flow General Purpose Lubricant Spray). This oil can be found at most hardware stores, but can even be found at some drug stores and supermarkets. Don't be afraid to spray too much, but have a paper towel handy to pick up the excess. Spray the oil into the keyhole and onto the bolt mechanism. Move the bolt in and out to allow the oil to penetrate into the lock. Do not take the lock apart if you are not confident you can put it back together.