Don't just grab the first lavender you see at the garden shop. French lavender is decorative and is pretty popular (i.e. popular because it's pretty) but doesn't have much scent to it. English lavender (Lavandula augustifolia or L. latifolia; a hybrid of the two called L. x intermedia is also available) is the type you want. You can take your pick as to the exact colour the blooms will be when they come out. You can get flowers in shades ranging from white to pale blue-purple (classic lavender colour) through to purple and pink.
Lavender also grows from cuttings, so if you have a friend who has a lovely patch of lavender, you can ask for a cutting or two. Lavender also grows from seed, but this can be a bit of a gamble, as lavenders hybridize with each other readily and come up with offspring that are nothing like the parent plant you took the seeds from. Always ask first before taking cuttings or seeds; we don't all have the chutzpah of one middle-aged lady who will remain nameless who was an expert at nipping off seed-heads from strangers' gardens within easy reach of the footpath.
Lavender likes the sun – the more, the better. While lavender is mostly pest-free, it is prone to fungus if you plant it in damp, shady areas. It likes well draining soils and doesn't mind stones (rather like good wine grapes). Taller varieties of lavender can be planted as a hedge (try L. augustifolia "Vera", which has nice silvery leaves and gets to about 90 cm high). Lavenders repel insects – except for bees and butterflies, which love the flowers – so it makes a good perennial for companion planting near the vegetable garden. One delightful suggestion for planting lavender this writer has come across was to plant lavender near your washing line where the sheets can flap against it, picking up some of the scent as they dry outside.
Lavender prefers an alkaline soil. An acid soil can be corrected with ash or lime.
Plant lavender in autumn for best results. If you're planting a hedge, put the plants about 30cm apart – you don't need to crowd them. One of the bonuses of a lavender hedge is that it doesn't lose its leaves over winter. You can't really call it an evergreen, but you could call it an "ever-silver" or an "ever-grey".
Lavender hedges should be pruned after flowering (never before, for obvious reasons – you miss out on the flowers). Dry the prunings and use them for kindling or on a barbecue – they will release the scent as they burn.