I am 12 years old, and I did not pray, so how do I atone for that?

The pen of accountability does not run except for the one who has reached puberty, based on the Prophet's (peace and blessings be upon him) saying: "The pen has been lifted from three: from the sleeper until he awakens, from the insane person until he regains his sanity, and from the child until he reaches puberty." Puberty is reached by nocturnal emission (wet dream), the growth of coarse hair around the private parts, or reaching fifteen lunar years of age. For a female, menstruation is an additional sign. If you had reached puberty and intentionally missed prayers, you must repent and make them up according to the four imams and the majority of scholars, due to the Prophet's (peace and blessings be upon him) saying: "The debt owed to Allah is most deserving of being paid." If you had not reached puberty, there is no sin upon you for the prayers you missed, but you will be rewarded for performing them, as a child's good deeds may be recorded for him, but his bad deeds are not recorded against him.