Garden design is a very personal thing and is often an expression of your personality. What I like you, you may not and vise versa. Some people like neat and tidy gardens where there are no surprises, others love the thrill of windy paths, lots of different plant material and not knowing what is around the corner.
To create a well designed garden, it is important to put the right plant in the right position. This means considering the cultural requirements of the plant. For example putting a full sun plant such as rose into a shady position isn't going to work, because the rose won't be receiving the right amount of sunlight for it to grow. The idea of good garden design is to follow this philosophy, using the placement of plants to create mystery, tension and surprise by using tricks of the eye, colours and textures.
If you are feeling overwhelmed about designing your garden, divide your space up and take it slowly, completing one section at a time. Don't start another part until you have finished the section you are working on and very soon you will have a beautiful garden. Remember gardens are ephemeral, it is a process that is for ever evolving. You never really finish.