That's a really loaded question. I probably have 100 domains in total, 5 full time staff members, another dozen I use part time, and loads of different niches. Rome wasn't built overnight.
If you're looking for starting out advice, go back to the fundamentals. Choose the right niche where there's money to be made, engineer value for people, learn how to sell it, and then learn how to get the right traffic to your offer.
One of the smartest decisions I ever made was stop coming up with "good ideas". One time out of pure frustration, I just made 15 test sites and said I would pursue the one that made money the easiest. It turns out one of the sites that was a pure afterthought converted at a $50-100 service (I outsourced it for $5-15) with 15% cold traffic. I was totally surprised, even downright shocked by it. Before that point, I probably would have pursued what I thought were the best ideas in my opinion.
Now I just follow the money where I reverse engineer what people want and then give it to them.
I hope that helps.