Defining what a 'native plant' is can be tricky. Many gardening books, including all too many wildlife gardening books, assume that any plant occurring anywhere on the North American continent before Europeans arrived is 'native'. But that's not a very useful definition if you want to restore or improve wildlife habitat in your own yard. Some plants from other parts of the continent won't grow here at all; others are terribly invasive and destroy habitat rather than improving it.
The next step is to find out which species occur naturally in our region. There are many books and field guides to help with this. My favorite is Pojar & MacKinnon Plants of the Pacific Northwest Coast. In addition to the excellent photos and descriptions, it shows in rough outline whether a species occurs at low or high elevation, only along the coast, or no farther north than Portland, OR. You'll have more success if you choose plants that grow naturally where you live. This gives someone in Seattle, WA a rather different palette to work with than someone on Victoria Island or Salem, OR, though there will still be a lot of plants that overlap.
There's another step that's worth taking-the plant community. Even within Seattle, there are many different kinds of habitats. Some areas (riparian) are near a stream and have soil with lots of organic matter that stays moist all summer; other areas (upland) are higher above the water table, with sandy soil that drains quickly and dries out thoroughly during the summer; there are meadows, and mixed deciduous woods, and deep dark conifer forest. These areas will all support quite different plant species. Monkey flower would be happy in the riparian community with little or no care, but would need frequent attendance in a full-sun upland garden. It's best to choose plants that match your location even on a small scale if you can. Go for walks in natural areas during all seasons; notice which species tend to grow together, and what the conditions are like throughout the year. You can't do better than steal great ideas from Mother Nature! For further help, join your state or province Native Plant Society. Most have local chapters that arrange field trips and workshops.
But wait, there's more! Once you know which plant community makes sense for your garden, and have chosen which species you want to plant, it worth thinking about where the parents of the plants you get came from. You want to find 'regular' plants if at all possible, not garden-variety named cultivars. Plants that have been bred for the horticulture trade are often not as useful to wildlife as a randomly bred plant of the same species. People like to breed for large flowers and fruits, long blooming season, varied flower colors, and more. But any of these changes that please human eyes can have unfortunate side effects for the wildlife that needs the plant. Those big doubled flowers may not produce any nectar for your rufous hummingbirds, or pollen for your mason bees. Or they may bloom too late for the bees, or be invisible to them because the color is yellow, not blue. Perhaps the fruits on that fancy berry shrub are too big for a cedar waxwing to swallow. If your primary purpose for the plant is to please your wildlife so that they can please you, avoid named cultivars. You'll probably save money, too!