What should I do to atone for my sin after having amused myself with a girl over the phone and promised her marriage without sincere intention?

Relations between young men and women must be severed immediately, and one must repent sincerely to Allah. Sincere repentance has five conditions: sincerity to Allah, regret for the sin, abstaining from the sin, resolving not to return to it, and that repentance occurs before it is too late (before death or before the sun rises from the West). There is no harm in marrying the girl if she is righteous and you desire her, for the Messenger of Allah, peace and blessings be upon him, said: "There is nothing like marriage for two who love each other."