You can only connect two devices if they do have a “protocol” in common.
A simple one to connect your PC to the audio is using a Y-cable (headphone out to RCA in).
This is a dirt cheap way to get started but don’t expect much sound quality wise.

Desktops with a discrete sound card can be connected using the line out.
DACs and a lot of receivers have a digital in (SPDIF). This might be electrical (coax) or optical but a lot of PCs don’t have a SPDIF out.
In this case a USB to SPDIF converter is an option.


Today there is a plethora  of USB DACs
The new generation supports sample rates up to 24/192 and uses asynchronous synchronization.

They have USB in and analog out.  

As there is no PC without USB and almost all amplifiers have analog in, this is the ideal bridge between the PC and the audio.

Some models combine this with a volume control and a headphone out, a DAC and pre-amp in one.

More details on how to connect the PC to the audio can be found here.