What should a person do who has taken money from his family, his sister's husband, his sisters, his friends, and the school shop without their knowledge, then denied it, and now desires sincere repentance and atonement for sins, while feeling a lack of success in his studies and fearing death while in a state of disobedience, and is also communicating with a girl via social media?

Allah loves those who repent and rejoices at the repentance of His servant. He forgives all sins, no matter how great, and transforms evil deeds into good deeds. Therefore, you must have a good opinion of Allah and supplicate to Him with sincere repentance, avoiding despair. The conditions for repentance are regret, cessation of the sin, and a firm resolve not to return to it. Regarding the rights of others, one must restore those rights or seek forgiveness. Part of sincere repentance is cutting off forbidden relationships and returning ill-gotten money to its owners, or giving it as charity on their behalf if reaching them is impossible. It is not a condition to inform the rights-holders of the details of the sin; it is sufficient to deliver the right or seek forgiveness by any means. You should seek help from Allah and exert effort in your studies, as sincere repentance can be a cause for success and achievement.