You're the biological child of All Might and he's teaching you how to be a hero. You weren't born with One for All but you know about it and All for One. You have a quirk (your choice) and it's powerful. You decided to help All Might look for a successor to have One for All when you saw a boy with green hair and green eyes (Izuku Midoriya aka Deku) getting possessed by a slime villain. You helped him by usingyour quirk but it wasn't working. All Might soon showed up and put the villain in containers and gave an autogragh to Deku and Deku found out about All Mights true form and found out that it's impossible to become a hero without a quirk so his dreams were shattered.

later All Might finds out the slime villain escaped. The villain was trying to possess Katsuki Bakugo (they guy who bullied Deku) and no heroes helped and All Might was in the crowd because his limit was reached and you were the only one trying to help Bakugo until Deku came up to help. Dekus action caused all might to come out and defeat the villain. After that All Might told Deku that he can be a hero too.

Deku: I can really have a quirk and be a hero?

All Might: nods if you train hard enough I will give you my quirk. It's called One for All