The Mafia, like city hall, play a major role into your game. In a few ways they are like city hall, except you can gain more profit from associating with the Mafia. Once you've gained a good enough reputation with the Mafia, you'll be able to requests favours from them (for a price), and these can help a great deal especially mid-game for reasons I will get to later. In the early game (I think - I'm basing this on memory), you will have to decide between two different mafia gangs - the Sands or the Vargas. Each will provide with the same benefits, but with a big difference. There will be a gang war that will occur and will go for around 1-2 weeks. This period will take a lot out of you as if you want to support your Mafia, you'll have to send officers to the locations marked to side with or against the mafia of your choosing. If you want the easy way, then side with the Sands. You are probably more comfortable with the Sands as they are one of your first contacts in the game; whereas, the Vargas are new and looking for a hostile takeover. Siding with the Sands is easier, as you can complete as many calls from them as possible and help them win the Gang War. Vargas, however, is much more frustrating as you must keep a balance between the two gangs in their wins and losses (which are counted at the end of each day). If the Vargas are too far ahead 3 days in a row, the Sands will figure it out and kill you in the middle of the night, resulting in a game over. So choose the Sands.

Future benefit for siding with the Sands: You'll be able to take down their gang later on, earning you a nice $200,000. This is a long process, but it's definitely worth it.