I think you have to get your inspiration from other gardens, particularly those in your area so you can see what does well in your soil type. Did you watch any of the Chelsea Flower show this year? Think of the colours that you want and then find things in different sizes in those colours. Crocus and other sites sell "ready made" border collections.
What I tend to do first is the "skeleton", so some evergreen shrubs or "backing plants" and then intersperse these with colour that will hopefully pop up at different times of the year - spring bulbs, summer bulbs, perennials and then annuals or biannuals.
Do you have lots of time to maintain or are you looking for low maintenance?
What sort of gardening style do you like - bold, modern, natural etc? Do you want year round colour?