Hi maryrose, You dont say where you are ?? but as a general rule they are best pruned immedeately after flowering and here in Cornwall UK, they are in flower now ! Depending on how small you want to keep it, I'd prune it all over by about 1/3rd, that will encourage growth from lower down. If you want to keep it really bushy and need lots of new fresh growth (ie. for cullinary purposes) as soon as the currents seasons new growth is about 6" long, pinch these shoots out again.
I would also suggest that now would be a good time to feed and that will help with the production of new shoots.
regards, Lannerman.