OK, so depending where in the world you are located, and budget available, I would suggest that it should be relatively easy to pick up a used amp.
If you have access to any of the hifi forums that are online, you might be able to source a good quality amp. And sometimes a dac hat for the pi.
There is, of course that 'bay and other sites, but they might be a bit tricky if you are just starting out.
I have always used seperates and have no knowledge of the amplifiers that are available for the pi, or the required software.
My computer speakers are 35 (or thereabouts) years old and I run a pi into them via a chip amp and a switch-box.
The better versions of pi amplifier also implement a dac, iqaudio is also a well respected name. Volume control is either from software or the addition of a rotary encoder.
If you have a pair of headphone you can start with just the pi.
Volumio and Moode offer Spotify-Connect, playback of local files and radio station playback.
Logitech media server is another, and my favourite (piCorePlayer OS).
I have dabbled with an "all-in-one" from here
There is an installation blog here
https://makingstuffwork.net/technology/ ... pberry-pi/
I use logitech media server and have built a Spotify-connect installation from instructions on the web
This is a good one
There is a bit of a learning curve with all these software installations, so a little patience and a lot of google is often the answer.
In fact, just google what you wish to do and see how many hits you get.
Hope this helps