Play golf for eight or more hours every day (or as much as you can manage). The saying "practice makes perfect" is certainly true for golf. Although no player is ever perfect--even Tiger Woods has his bad days--the best players practice as much as possible.
Find a golf course with a professional instructor on staff and take golf lessons. A pro can make a huge difference in your scores by showing you proper technique and helping you work the kinks out of your game. All serious golfers have some instructor that helps them.
Enter in competitions. Many golfers that can play well by themselves will choke under the pressure of competition. Playing well under pressure is an essential skill for any professional athlete, and the best way to get better is to compete as much as possible. Start playing in all the tournaments in your area--even small ones are good practice.
After lots and lots of practice and at the recommendation of a qualified professional, apply for the PGA or Nationwide tour. The typical path for PGA qualification is the PGA Q School (qualifying school). This "school" guarantees the winner of the qualifying tournaments a PGA card. The Q school can also qualify you for the nationwide tour. The nationwide tour is like the minor leagues for the PGA tour in that all the golfers there are pros but are not quite playing at the level of the main PGA tour players. Go to the tour website for applications.