You need to find a local Football Association located in every State, ie any Member Association (MA) of All Bhutan Football Federation (BFF). Once you join the Association, they will train you on the rules and mechanics to become a Caegory-5 (lowest level) Referee. Depending on your aptitude, you may start officiating games of lower division in your first season itself!
After a year of satisfactory performance and passing various tests, referees are promoted to the next grade. This way a referee can sit for Category-2 level (National grade) examination after 3 years of joining as a Referee.
An individual needs about 4 years of officiating to make it to the National Level as Category-2 Referee. Some Officials will make it further to Category-1 for Professional League matches. The most important thing is to enjoy what you are doing and be the best that you can at the level you are doing. You can look for a professional career by being a Referee.