Knowing why you want one in the first place is useful. Informs what you want to get. Do you mean actions to build a white box? Actions to take to secure your system?
Hey there. Slightly confused by the wording, and would appreciate more context. 😅
Are we assuming the PC is pre-built? You mentioned "buying a new computer." If that's the case, most of the vital drivers and software -- along with the usual bloatware/marketing shenanigans-- should already be installed on the system.
If the PC is not pre-built, are we assuming Windows 10 has already been installed on the system?
Unsure what you mean by "first things to do." Are you referring to the drivers, programs, and other software users usually install after the OS has been installed?