-Create various NPCs in the family, as well as the other families
The first thing I would recommend for NPCs is the ubiquitous name lists:
Italian-Americans tend to have anglicized versions of Italian first names, such as Tony, Joe, Mike, Paul, etc. for men and Marie, Phyllis, Josephine, Teresa, etc. for women. For Italian last names simply crib from your favorite Mafia themed movies and television shows. However, another easy cheat is to Google Italian towns and cities and add “De” to the front of the place-name for an easy Italian Surname: for a real-life example, my Grandmother’s maiden name is DeMarco.
Now, as far as the actual NPCs go: follow the RPPR Golden Rule; Steal Blatantly. However, I would advise you NOT to steal from mafia T.V. shows and movies; your players will be looking for this. Instead, steal characters from other kinds of shows, change their names, and work them into the context of your campaign. For example, the crew of Serenity from the show Firefly would make a great mafia crew-just change the names of the characters. Another great source would be the show Babylon 5: virtually every character from that show makes a great NPC for any game; I have used them myself in my Iron-Age DnD game.
-Design various events that will happen whether the players do anything about it or not
For this list of events (that could easily double as adventure hooks), I would make a list of events divided into two categories: Major Events (that would indirectly affect the PCs but have reaching consequences) and Minor Events (that would directly affect the PC, but are easily resolved, i.e., adventure hooks). I would create, say, 5 Major Events and 10 Minor events, depending upon the length of your campaign.
Major Events would include: The Boss of a Rival Crew is assassinated; the local DA arrests a major Boss on a RICO charge (bad news for any crew), etc. Minor Events could include: the girlfriend of a PC announces she is pregnant, the brother of a PC is a gambling addict and is in 40 grand deep to the boss of a rival crew, a rival gang (not mafia but, say, Russian, Yakuza, or Irish) moves in on the PCs turf, etc.
-Have various jobs for the players to do.
Most members of Mafia Crews are basically left to their own devices; they must simply give a weekly “cut” to the crew boss, who in turn gives a cut to the Capo, and on and on. This element could introduce a nice sandbox element to the game as the PCs engage in various attempts to “earn,” collecting extortion money (protection), loan sharking, gambling debts, and a little B&E on the side.
As far as specific jobs given by Bosses and Capos: these would mostly include collection of overdue debts (“Frankie the Scab is in ten large and he ain’t paid up in a while. Go convince him to quit being a douche-bag and cough up some of what he owes.”); Babysitting (body guarding various NPCs); and, if the PCs are real toughs, assassinations.
-Have a list of traits for the PC creation
I’m not exactly sure what you mean by this. Are you looking for GURPS-style advantages, disadvantages, and quirks, or are you simply looking for unique personality traits to assign to the PCs? Either way, the aforementioned GURPS mechanics are a good place to start.
-Have main locations in the city for the players to become familiar with it
Locations would fall into three broad categories: businesses owned by the PCs and their associates that would double as meeting places; ‘Neutral’ territory for meetings with other crews and bosses; and ‘mark’ locations- places where the PCs collect protection money, engage in B&E, fight other crews, etc.
Examples of businesses/ meeting places would be Bars, restaurants, delis, bakeries (Italian-Americans are big on food), etc. The same would apply to the ‘neutral’ territories. The ‘mark’ territories would be anywhere the PCs would be able to pick up easy cash; mostly businesses but residences would do as well. One great resource for this aspect of the game is the out-of-print Shadowrun Book Sprawl Sites, however any floor plan will do- you can even reuse old floor plans by adding “new paint” to them.
I hope these general tid-bits help. If you would like specifics of anything above, feel free to relpy either on this thread or via PM. I will be happy to provide what I can (time permitting). Good Luck with the game!