Select a variety that is hardy for your area; Angustifolias (the English types) being the hardiest. The lavandins are hardy also. The stoechas (spanish types) are usually hardy for our NW winters. If you are in a gardening zone below 8, the spanish varieties would be good container plants for you, since you could enjoy them inside during the winter months.
Plant your new lavenders giving them plenty of room to spread out 2-3ft. They love sun and good drainage.
Mix chicken manure and bone meal (about ½ cup of ea.) well into your soil before planting. If your soil is hard clay, mixing some sand in will help to loosen the clay. If the plant is rootbound, loosen the roots before planting. If the plant is tall or spindly, cut back to encourage more lateral vegetative growth.
Using sand for mulch around the plant keeps the plant clean and reflects light back up into the plant, thus creating more heat. More heat creates more fragrant blooms.
Prune your plant in the fall to 2/3 of its size, leaving a couple of inches of green above the woody stems. This may seem severe, but they will respond very well to this "hair cut".
When your lavender has blossomed, the flowers can be picked for many uses. If you desire a fresh bouquet, pick the blossoms when ¼ to 1/2 of the flowers are open. If picking for sachet bud, pick when 75% to 100% of the stalk is flowered out.
You can dry the lavender from your plants by cutting bundles of 100-150 stems per bundle. Wrap a rubber band around the bottom of your bundle and make an s-hook out of a paper clip and hang upside down in a warm, dry, dark place. Drying will take a couple of weeks depending on the weather.