How can I monitor office staff?

You need to introduce a system to monitor the performance of work by staff. Also important is the motivation system. All people know how to count money and their income

You can use Refog Keylogger  https://www.refog.com/keylogger.html to monitor employees 
It's a simple program. This program is easily installed on the computer and does not require special skills. 
Refog Employee Monitor records and monitors your employee's activities and provides you on-site and remote access to employee's logs and computer screens in real time. 

Ofc you can do it, by camera for example

The best option is to hire employees who do not require frequent and close monitoring. Control consumes an excessive amount of time for both the boss and his subordinates, and, worst of all, the quality of work is still mediocre. Also, excessive control in the office has significant disadvantages:demotivation of employees, demotivation of the manager and the most dangerous low performance of the company as a whole, and in the departments in particular.To begin with, I advise you to motivate your employees by providing them with a comfortable place of work. You can try to use shared office space, and understand in which type of office the work is done as efficiently as possible and in which office it is much more convenient for you to monitor the work of each employee. The most excellent type in my opinion is an open office divided by small partitions. In this office you can see each employee and what he is doing without causing discomfort to employees and not diverting them and the employees themselves have their own personal workspace that they can arrange themselves.