I installed DHIS 2 without the sample data and I don't know how to create a root unit. Could you tell me how to create a root unit?
I assume you mean a root organisation unit? You can do that from the organisation unit app:
Organisation Units app
-> Organisation Unit
Add new button in the upper right corner.
The first organisation unit you create will be the root of the organisation unit tree.
However I would recommend using the Sierra Leone demo database from https://www.dhis2.org/downloads (under "sample data") to get to know DHIS if you're new to it. Setting up DHIS from scratch can be a lot of work.