Shiva is a time when we reminisce, remember, recapture memories of a loved one. As such, what we usually do during a condolence call is to listen to those memories that the mourner wishes to share or to talk about other subjects initiated by the mourner that may have nothing to do with his or her loss.
It is traditional to not knock or ring but rather just to enter a house of mourning, so as not to bother the mourners. Many Reform Jews do not observe this custom today, but it is a good idea to keep it in mind and try the door before you ring the bell when paying a shiva call.
As you enter the house of mourning, a member or friend of the family may meet you and usher you into the living room. It is customary to wait to speak until after the mourner speaks. But, once you are acknowledged, all you need say is “I’m sorry.” That simple phrase, a touch, a hug will mean more to the mourner than you can ever know. Usually, you need not stay more than thirty to forty-five minutes. During your visit, supporting, listening, and responding to the mourner should be your primary goal.
Except for food, it is not customary to bring anything with you to the house of mourning. Again, your presence is the main thing. If you wish to “do something,” make a contribution to the deceased’s favorite charity or synagogue fund. A particularly meaningful gesture for many Jews is to plant trees in Israel through the Jewish National Fund.
Source: Rabbi Daniel B. Syme, The Jewish Home: A Guide for Jewish Living (URJ Press, 2004)
See also: Preparing for a Jewish Funeral: A Checklist, The "New" Jewish Funeral, What to Expect at a Jewish Funeral, Preparing for a Jewish Funeral: A Guide.