Database Threats and how to Prevent them

I am tasked with security of the systems database, as the previous operation lacked securtiy and left gaping holes in the system. The Systems information was leaked, and clients refords were leaked as well as workers salaries. I need to find One technique or strategy to prevent this information loss, and propose how we will prevent this in future. I am relitivly new to DBMS, can you please help me.