How to repent from all the sins a person has comitted?

Can you please tell me how can I ask forgiveness from all the sins I have committed, and will shirk also be forgiven by that because I have comitted many shirk when I was between 12 and 13 and I was between puberty in past, will my sins be forgiven without anything because I was young and even if it will forgive my sins I will still repent, I wanted to repent first, but I could not gather sincerity, that's why, I watched videos of mercy of allah and now slowly I am regretting those sins, can you please tell me how can I ask forgiveness? Please Answer? And if I don't have enough sincerity will I be still forgiven? I want to repent but I can't gather enough sincerity, can you please give me a dua for this.