So, you've got the circle set up. The Quarters are there, your bubble of protection is secure.
Now what?
Now you invoke the Deities. Remember? The reason you're doing this in the first place? You can light their candles as you invoke them, if you like.
Show the Lord and Lady respect. They love you, after all, and you presumably love Them.
How you call the Lord and Lady depends on many factors:
If it's a Sabbat, then you want to call the Diety(s) that best fit the holiday; if it's a full Moon, then you may want to call a Deity with lunar associations, such as Diana or Selene. If it's no special date at all, then that's fine too; just check your timing. Is it the right moon phase for this type of ritual? If your ritual is to honor the Lord and Lady, then you should call upon the Deities that you feel most comfortable with. In fact, your ritual should always be done in a fashion that you are comfortable with. It's hard to be in Perfect Love and Perfect Trust when you aren't okay with how your ritual is set up.
Many Wiccans choose a pantheon (set of gods and goddesses from a specific culture) to work with. This is often extended to a God and a Goddess. Others, of different traditions, would work with the God and the Goddess of their particular tradition. There is no right or wrong Deity to call. There is such a thing as calling a Deity that may not be appropriate to the ritual you are holding, though, so I recommend that you study about the Diety(s) that you are calling before you call them.
An invocation for the Lady might go: "Lady, Goddess, Great Mother, I call to you and ask you to join me in my rite." Now, that's a short one. Very short, considering how long some invocations are--but it still does nicely. If you have a specific pantheon, add the name of the Lady in that pantheon to the Lady, Goddess, Great Mother part. If you're doing it for the God, whack off the "-ess", call Him the Lord and Great Father, and substitute God names. If you want to invoke specific attributes of the God or Goddess, mention them. Juno Moneta for money, for example, or Venus for love, or the Sun God for strength.
You get the picture?