What is the expiation for the sin of someone who engaged in masturbation after an indecent conversation with a girl via an electronic game, and then regretted his action?

The expiation for a sin is through sincere repentance, which includes abandoning the sin, regretting it, and resolving not to return to it. Hasten to repent to Allah Almighty from sending lewd messages to the girl and from masturbation. Do not despair of repeating the sin and repenting, for repentance is accepted, Allah willing, "Indeed, Allah loves those who repent and loves those who purify themselves." And from the sincerity of repentance is to avoid the causes of disobedience and to cut off the paths that lead to it.