Please tell me how to set up simple computer system

I think any recommendations need to be based on more information. But it doesn't seem likely that there is a high end audio system involved here. Otherwise, discussions of how much difference JRiver, vs iTunes, vs shutting down background jobs... vs a dedicated streamer is going to make. 

It is sounding like connect USB to some input device and choose the output device on the MAC. 

I agree we need to know more to be really helpful. Things that are important to know include:

1. What other components are involved so we can determine where on the spectrum this solution needs to sit.
2. How computer savvy the person is and/or if they have a friend who can do some technical heavy lifting.
3. Where streaming fits in with other medium; will it be a primary source or more one to explore new music?
4. Will the streamer also stream .flac files ripped from CDs? If so, how many CDs worth?

I agree, old/new, it’s just bits to the DAC

I learned about using USB out of my PC to a Topping D10 DAC, I stream lots of audio and music videos in my office, the most important was the physical re-arrangement to get great imaging.

https://www.amazon.com/dp/B08F54TT79?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_1&th=1

I needed a USB 3.0 extender cable to put the DAC on the shelf below my amp

https://www.amazon.com/dp/B00P0ES0YE?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_2&th=1

Sit dead center of speakers and monitor