Be discriminating. As much as you'll want one of everything, hold off on buying plants just because you like them, or your garden will look like mulligan stew.
Go with nature. Look first at the palette of native plants around you. Design with and around nature.
Give plants enough room to grow. Think of the plant's ultimate size when you plant. You wouldn't want your teenager sleeping in a crib.
Plant for all-season interest. Include plants that look glorious in several seasons (as beautiful evergreens do) and ones that have seasonal appeal (flaming foliage in autumn, beautiful bark in winter).
Build a reference library. Ask for good garden reference books for birthdays and holidays. Start with the Sunset Western Garden Book.
Learn one genus at a time. Pick a plant group that you like (Mahonia, for example) and learn everything you can about it. Then move on to another genus.
Make daily rounds. Never miss a day walking through your garden to tell the plants how beautiful they are. It makes a difference. A garden is like a love affair―it thrives on attention.
Cherish winter. This is the time when the beauty of nature will do the most to cheer you.