The first step of Tawba is feeling guilty about your sin. You must acknowledge the sin and feel ashamed of it and to stop the sin immediately and make resolve not to return to it. If you had harmed someone then you need to seek their forgiveness. This is sincere Tawba and have hope that Allah has forgiven you.
Remember, Allah forgives in abundance and very Merciful.
"(And remember that) Allah's acceptance of repentance is only for those who commit evil out of ignorance and then soon repent. It is towards such persons that Allah turns graciously. Allah is All-Knowing, All-Wise." (Sura Nisa 4:17)