An easy mistake to make with a new garden is making it too complex. In the nursery everything looks great. The plants are small and often very cute, until they start to grow out of control, but of course by this time they're in your garden and destroying the picture you originally had in your mind.
Great looking gardens are often the simplest in terms of complexity. Start the garden plant out by knowing the size of the space. Measure it out. Second, study the types of plants to understand the're growing habits. For example are they side spreaders or will they grow tall. Imagine in your mind what they'll look like with other plants/shrubs around them. Consider the amount of space you need to walk through the garden, to weed it and add other things that may enhance the concept you're trying to create.
Choose low maintenance plants or at least ones that are easy to prune. Maintaining a balance as plants and shrubs grow can be hard so early thinking on these matters will help you to enjoy your garden for years to come.