Ideally, a private lesson with a professional guitar teacher heads the list. It might be the most costly initially, the cheapest method when you factor in the time saved because of the structured way a teacher would employ. Professional instructors will not allow you to develop bad habits which may prove hard to change once they become ingrained.
Another method is to join a learning group of people learning to play. The third method is you learn online, either privately or by buying an online course. The last option would be to learn from books, cheapest also the most challenging way to learn to play the guitar. Take the time to explore all the options available to you.