Depending on the organization (implementing ITSM) as well as process maturity, there are many elements that must "fit" so that ITSM is implemented (which could be very broadly interpreted, anyways).
So, here are few items that must be in place, adapted to the services you provide and managed:
Each of these elements must fit for purpose, be mature and be managed.
Here are few articles that will give you a hint how to start:
Ready, steady… go – Starting ITIL implementation https://advisera.com/20000academy/blog/2014/06/10/ready-steady-go-starting-itil-implementation/
7 effective strategies to gain employee buy-in for ISO 20000 implementation https://advisera.com/20000academy/blog/2017/09/05/7-effective-strategies-to-gain-employee-buy-in-for-iso-20000-implementation/
This free webinar can help you speed up the implementation "How to use a Documentation Toolkit for the implementation of ITIL / ISO 20000" https://advisera.com/20000academy/webinar/how-to-use-a-documentation-toolkit-for-the-implementation-of-itil-iso-20000-free-webinar-on-demand/