Lavender seems to be one of those plants that is happy if it seeds itself and stays put where it chose, it does really well. Just ordinary English Lavender will make a hedge, It needs to be trimmed after flowering and then trimmed over again in the autumn. Keep away from old part of the stems as it does not like to be cut back too far. Maybe a bit of compost mixed into old potting soil as a kind of mulch. It does get leggy if not trimmed.

Also if it is growing in the ground dont water it except in exceptional draught condtions they prefer it very dry, I dont mulch mine either.

Depending on the size of hedge you want, Lavandula 'Munstead Dwarf' or 'Hidcote' are good growers and will both form good hedges if planted at the right distance. As Dorjac says, they should be clipped back after the main flush of flowers (usually by end of June) and again at the end of August, taking some of the new growth at the same time to reshape the bush and prevent legginess. they prefer full sun and lightish soil, but will grow on clay if its been improved a little. Keep watered during dry spells after the initial planting for a couple of months.