lightly trim the old flower stalks back do not cut into the old wood as it will probably die ! i would wait till later in the year

do you have an idea when I should think about pruning it?

Lavendar should be pruned after flowering in July/ish time. The idea being that you gather the flowers for the nice scent, I put mine in the wardrobe in a paper bag. However, to tidy it up this year just trim off the old flower stalks as Pauchy says.

O.K, thanks for the advice

Clip over twice a year, taking about an inch of the foliage with the spent flower stems - this usually needs doing around end of May or June, and again in about August, early September. If you haven't done it this year, then you will need to clip it a bit in Spring, but wait till April, when its properly warmer.