What is the common words I must use in a formal letter?

It's not just the words that have to be formal, everything about the letter has to represent professionalism and formality, starting from the layout to the signature.
For example, for the greeting you can use : Dear Mr./Ms.
, also in the body of the letter you have to be very practical, avoid expressions personal ideas or feelings, and lastly use a respectful formula for concluding the letter such as : Kindly accept my best regards, yours respectfully ...etc.

Hi - Formal letter is formal when formal courtesy words are used in to it, we must use formal salutations showing respect to the other person, there is not one single word that can be written here, it is a vast langague so you can use as per your choice....

Dear Mr/Mrs.
........: yours sincerely,

An official letter is cool of course, but the way a free letter surprises me, nothing surprises me anymore. A good example is this letter https://www.puckermob.com/moblog/an-encouraging-letter-to-my-love-in-jail/ . There is the text of a letter from a man in prison who writes the most beautiful words to his beloved. When I read this text I just cried. 

I believe you must start with the following narration Hello !! Greetings of the day Sir/Madam !! that will impact some good impression towards the viewer to encourage the applicant to further move into superior step.

Respcted>>>>>> Dear,,,,,,,,,,, b happy alwyz :)