Just bought a 2013 Kia Rio LX. Love the car, hate the audio. Need noob pointers.

The stock audio experience is lousy. I am looking to spend $300-500 to start, maybe more later if needed. Don't care about upgrading the factory stereo unit, it plays cds and hooks to my cell for music...good enough. I thought maybe 4 new door speakers, maybe a couple of 6x9s in the back...might need a good cheap amp too? I haven't done anything with car audio since mid 90s, no idea where to begin. More of a techno or industrial buff than anything else, if that matters. Any ideas, pointers, sites to learn more, etc.? Much appreciate the help!