@job123 RepRapFirmware needs to be running on the Duet for you to do anything with it of course, including connect to its API over HTTP. On the SBC you do not need any specific software form us.
The API documentation is as linked before:
https://github.com/Duet3D/RepRapFirmware/wiki/HTTP-requests
This thread may be of interest to you:
https://forum.duet3d.com/topic/31198/reprapfirmware-python-api
This project may also be of intrest.
https://github.com/AndyEveritt/DuetWebAPI