If you have well drained soil your newly planted hedge will take off. This three year old lavender plant above is in a one foot deep soil pocket on top of two feet of limestone lumps, so it is pretty well-drained.
You can buy lavender plants or take cuttings from an established plant. The cuttings will establish themselves in a few weeks and in a year will be about six inches across. You will not get many flowers this first year as the plant puts its energy into growing.
In its second year a lavender cutting will easily grow to a one foot diameter plant and flower extensively.
There are two types of lavender. The more common, taller type is the one you need to plant for a lavender hedge. It will grow to a height of nearly three feet eventually. Lavender plants tend to spread more than they grow upwards. If you are planting a lavender hedge space the plants about 18 inches apart so the individual plants can grow together to give a continuous hedge without any gaps in it.
If your soil is clay-based then build a ridge about a foot high to plant your hedge on. Mix grave with soil/compost to give you the drainage benefits of a raised bed.
Lavender does not need a particularly fertile soil, so just throw a handful of organic fertiliser such as bone meal around each plant once a year.