Start by doing a bit of reading on the Myers-Briggs personality typing system before you attempt to find your particular combination of functions. Know that it is based off of the famous psychologist Carl Jung's personality theory.
Understand that the Myers-Briggs system utilizes a combination of four letters that correspond to a set of functions. The letter that corresponds to your type is your preference for one particular function over another. Start with figuring out whether you are extroverted or introverted. Note that this refers to whether you are energized by spending time with people or more energized spending time alone.
Decide whether you prefer sensing (gathering information using your five senses, and preferring to trust those senses to help you make decisions) or intuition (the synthesizing of information using your instinct or a culmination of perceptions you have gathered that don't necessarily rely on your traditional five senses). Figure out whether you prefer straightforward sequential planning or if you have a tendency to build ideas from disparate concepts.
Next determine whether you prefer thinking (trusting your ability to logically and sequential think) or feeling (trusting your ability to assess your emotions about situations) to make a decision. Think about what you do most of the time as everyone has a tendency to utilize each of the eight functions at some point.
Finally decide whether you prefer judging (the tendency to plan and require solid and finalized plans) or perceiving (the tendency to like open ended decisions and more spontaneous decision making). Combine the first letter of each of the four preferences to determine your Myers Briggs personality type.