Damned are delicate things. Some Damned seem to 'die' and their bodies remain without ever reverting to Maggot stage. Regenerating or intially generating Damned are called 'Maggots'. Therefore, there's no problems with most characters starting out, at least, in Pandemonium City. See the realm maps for positions of the more major Ruins.
Most capitol cities are the only sources of population in the Realms.
Depending on the character you're going to play, you need to read one or more, if not all, of these sections before you begin. If you plan on being a new-born Damned, bother with learning the New Damned section ONLY . If you're an experienced Damned, read the New Damned section plus the Experienced Damned section. Feel free to read all the sections that you'd like, but ICly, you're only expected to know the sum total of the proper sections you need to know for your character. We don't expect a new Damned to know about the city of Deca, for instance, or anything much about the individual Princes.