Whatever your relationship may be to the person who invited you to attend a shiva, keep in mind that there are some basic guidelines for making a shiva call:
Decide when to visit – Listen for an announcement at the funeral service for the times that the mourners will be receiving guests. Usually the options are immediately after the funeral, around the minyan (prayer) in the evenings and mornings, or during the day.
Dress appropriately – Most people dress as if attending a synagogue service. Depending on the area of the country, more informal dress might be just as appropriate.
Wash your hands – It is traditional to ritually wash your hands upon returning from the cemetery. This reflects the belief that contact with the dead renders a person "impure."