It is a fascinating question:
From where to start the digital transformation journey?
There is no one answer to this question.
You could say start by refining Business Strategy and goals or,
Develop an Enterprise or Business Architecture or,
Documenting current business processes and future business processes or,
Change management assessment, or,
Undertaking business health checks of capabilities or,
Enterprise Business Analysis, or,
Developing a Business Case, and so on.
There are pros and cons to whichever way you decide to start with.
You can keep thinking, consulting, seeking advice and sitting on the fence to decide the perfect start. Alternatively, you can just start by reviewing options and picking the most sensible one.
When our house is messy, we can spend hours thinking about where to start or start from one corner and keep going.
The progress is in initiating the work, iterating, learning, adjusting and keep on going! So, start from the most logical place (which may not be perfect, and it is okay)!