Learn to differentiate between shrubs and trees
It's not always easy to tell the difference between a tree and a shrub, especially since some shrubs can grow large enough to tower over smaller trees. One way to make the distinction (short of consulting a reference book) is to look at the main stems. Trees are likely to have just a single trunk, while shrubs typically have several to many, often equal in size. Also see how the foliage grows: trees usually carry their leaves on branches growing from points fairly high on the trunk, while most shrubs carry foliage right down to the ground.
Hundreds of shrubs are good choices for the garden. To narrow the field, start by considering only those suited to your climate. Next, think about the role you want each shrub to play. If you're looking for a plant with a strong structure to act as a "filler" between taller trees or buildings and a stretch of lawn, try one of the many large, broad-leafed evergreen sorts, such as sweet olive (Osmanthus fragrans). For an informal border to edge the back of the garden or a formidable barrier against wind or intruders, start by reviewing the Shrub Sampler. Maybe you want a shrub that will grace the garden with exceptional blossoms each year, such as rhododendron or lilac (Syringa). Or perhaps your goal is to find a shrub with interesting leaf color, such as gold dust plant (Aucuba japonica 'Variegata') or variegated tobira (Pittosporum tobira 'Variegata'), to complement a planting of flowering perennials.
Setting Other Plants among Established Shrubs
Planting annuals and perennials among established shrubs can be tricky. If you're constantly setting in and removing plants, the shrubs' root systems will be disturbed. Tough, carefree shrubs may not be bothered by such intrusion, but less rugged individuals may be weakened. In some cases, it's the "intruders" that suffer--the shrubs take most of the available nutrients and water for themselves. If you want a mixed bed of flowers and shrubs, consider choosing perennial flowers; they'll thrive for several years before needing digging and dividing. It's also a good idea to set out fairly small plants--those in 4-inch or 1-gallon pots. These require smaller planting holes and so disturb shrubs' roots less.
If you're adding a ground cover near or beneath shrubs, make sure that both plants have similar water needs. For example, a thirsty lawn isn't the best companion for shrubs that prefer dry conditions.