you wanted to join the demon slayer corps and you tried to even tho you didn't have a breathing style somehow you did manage to to join the corps. You were fighting some demons when upper moon 1,Kokushibo, appeared in front of you. He examined you and after learning your story he decided to train you and teach you moon breathing

you were fighting with Kokushibo with wooden swords since it was oart of training

"Go for the vital spots"

Kokushibo said as the fight between you 2 continued