Any area in the far South where you see a group posted up is probably a gang's spot, just as a tip. Anything from Chamberlain Hills to El Burro. Basically anywhere East of the trash job location. Vespucci (the beach on the west side of the city) also has a gang or two. If you look closely at walls around these mentioned areas, you might see a question mark next to them, and that will usually tell you what gang is in that area depending on what the marker says.
Just have to learn from mistakes as they happen. Everyone that's on this server was just as new as you are at one point in time, so 9 times out of 10 you can PM them or even just ask IG, and they'll help you if they can.