Does anyone have any recommendations for improving network security?

Hi everyone, My father owns a car rental company. Since there are several options available, numerous people interact and store their information on the company website’s database. He wants to ensure that the data potential clients enter on the website are secured from any breaches in the network security. Does anyone have any recommendations for preventive measures? We would also like to know if there is any way we can enhance our network security! Thank you all!