remove the flower stalks when they fade in colour and trim the lavender in april.
dont cut into the really thick wood of it will die,a gentle trim will be fine.put the liitle cut off bits in a pot and grow yourself another plant for free.

I just make mine look tidy. I have lost to many lavender plants to poor pruning. I don`t think you you can really have them looking like new plants in their first year in the garden. You have to accept them as they are and settle for the beautiful fragrance every time you touch them