The one that smells of lemon - is that the darker one of the two by the steps? If so then it might be either lemon balm or lemon verbena - google both and see if either suits.
As for picking the herbs - I just pick a sprig or several, as much as I want. You won't do them any harm. The Golden Marjoram will have flowers on it soon and the bees will love that one too.
In the late autumn, when you're tidying the garden for the winter, you can cut them back then if you like, but I prefer to leave the top growth on to protect the crown during the winter, then cut them right back in the early spring.