Starting your Business -- your best bet
The best wealth building strategy that most billionaires applied was to start their own business. Finding a service that you can deliver to others at a profit, and that can be scaled easily is the hallmark of a business whose worth will grow exponentially. Examples include restaurant businesses such as McDonald's, or Internet businesses such as Google, each of which serves billions of people each day. Most successful businesses that have made its founders into billionaires scale very well, and scale profitably. Study multiple successful businesses -- and seek out their competitive advantage, and how the companies worked hard at sustaining and maintaining that advantage.Sometimes new and innovative businesses in areas of technology and healthcare or renewable energy (as examples) can be bought out at a very high prices. Hotmail.com was acquired at $400 million in the dot.com boom time, and more recently, YouTube.com was acquired for $1.65 billion -- in each case because they provided innovative new service in the field of Internet. Large technology corporations such as Cisco systems, Oracle and others continue to acquire technology companies for several hundred million dollars that offer unique, niche services that can scale and be provided at a profit. Similarly, pharmacy companies are also very active in seeking out biotechnology startups who create new drugs, or technologies that can speed the search for new drugs. And as mentioned before, renewable energy technologies are getting more attention every day -- and it won't be long before significant acquisitions in this space begin to be made as well.
Stock Market Investing -- a difficult route
Warren Buffet was the first billionaire to get there entirely through stock market investing and this has popularized this idea. However, Warren Buffet also did this as his own partnership, running it as his own full-time show. Subsequently, he used leverage through the insurance business to get the cash he needed to invest in the stock market. All this, combined with his stock picking abilities drove him to become a billionaire (many times over). Thus, as the story might make it clear, apart from stock picking skills, you also need scale -- scale of money to invest and the courage to make large bets. Most endowment funds with money to invest do not put all their money in a few stocks, but instead buy the market (in varying allocation strategies). However, the path to becoming a billionaire is to place large bets in a few positions and ride the wave.Running hedge funds is another path to the billionaire status -- as you get more freedom to operate than a regular mutual fund, and you get a large slice of the profits you generate. Becoming a hedge fund manager does require substantial credentials on your part -- but then, if getting there were easy, everyone would be doing it.
Social Influencer
Some billionaires, such as Oprah Winfrey, have reached that status through their power to influence people's decisions and choices. Corporations value such influencers, and are willing to go far to either get them to sponsor their product, or otherwise create avenues through which their services can be associated with the influencer. Usually, money generated through sponsorships and other means continues to keep you in the millionaire club (though several hundred million) -- and to become a true billionaire, that money has to be invested wisely -- whether that be real estate or other investments that are safe and generate a steady return.Choose your course early, and wisely.

