Lavender is primarily a Mediterranean plant, growing well in regions of France, England and Italy. In the U.S., Washington State and Oregon grow much of the Country's lavender.
That being said, lavender can be grown in many places under the right conditions. With the right type of lavender and working within your area's climate, lavender can be grown outdoors in zones 5-6 and higher.
Cold or wet climates can enjoy lavender as an indoor potted plant.
Having said that, Lavendula Stoeches and Dentata are more fragile plants, frequently grown as annuals or as indoor potted plants in all but the most temperate areas (zones 8-11).
Lavender likes to grow in a warm, dry, sunny climate that has a cold winter. Too much water or humidity can cause problems such as fungus and root rot, killing the plant. Lavender can tolerate reasonable amounts of dry weather and prefers to have dry "feet".
Lavender prefers well-drained soil that is free of weeds. A mixture of sand, gravel, compost, stone, etc. helps to keep it's roots at the correct level of dryness. The planting hole should have a fair amount of compost and sand mixed in to ensure good drainage. Mulch can be crushed oyster shells, crushed stone, peastone, etc. A natural material that reflects light works great. Alternatively, some people use a fabric or plastic weed block.
As a seedling, lavender needs water as frequently as an other plant, letting it dry in between waterings then giving it a deep drink. This should be done until the roots have established themselves. After that point, Mother Nature should take care of the watering, except in cases of extreme drought when you may have to water from time to time.
A small amount of balanced fertilizer may be used when planting and once er year in the spring if so desired, but is really not necessary.
Spittle bugs may be found on occasion on lavender and can be simply washed away by using a firm spray from a hose. Most common garden pest and vermin are repelled by the camphor smell of the lavender, and thus can be used around the garden to keep away some pests, including deer.
Depending on your region, your climate and the care you give your plants, you may be able to enjoy your lavender for 15 or more years!