Need help setting up bookshelf speakers for my new PC

So I built a new pc and decided to buy some decent speakers for the first time. I have never bought somewhat decent speakers before so I have no idea how to set them up or what should go with them. I want to be able to set up 5.1 surround sound later, so my questions are about what else I need and how I set it up.

So a couple of things on the PC I built and the speakers I have:
-Z370-E Asus STRIX motherboard
-GTX STRIX 1080 GPU
-Polk Audio TSI100 bookshelf speakers
-Polk Audio PSW10 subwoofer

So the first question is which DAC should I get? I have about $100-150 left to spend, and I want to get a DAC and an amplifier. I want the amplifier to allow my headphones to be plugged in so I can switch between the headphones and the speakers. Should I buy a DAC and amplifier separately? Or should I get one that has both? If you guys could give me a couple of suggestions on what I should get that would be great. I don't know much about DAC's or amplifiers so the more info the better.

The second question is how do I set this up? I read some articles that said I should plug the speakers into the sub, and then the sub into the DAC. Other articles said some other things and I'm pretty confused on how to set it up. Can anyone explain how I should plug the speakers and the DAC/amplifier into my PC?

The last question is what type of cables should I used to get the best type of audio? USB, Optical audio, etc?