What must a person do to atone for the sin of taking small sums of money from citizens and using them for work or personal needs, after having repented of this act?

Taking these funds is forbidden because they are either bribes or gifts to employees, and both are prohibited. Bribery is one of the major sins, as evidenced by verses and hadiths that prohibit it and curse both the receiver and the giver. Gifts to employees are not permissible except with the employer's permission. Your duty, along with repentance, is to dispose of the illicit money by giving it in charity if it is impossible to return it to its rightful owners. It is recommended to increase acts of worship.