First step, and its easy and free: Listen to the latest C10 Talk podcast. Its all about audio. Now I may not 100% agree with some of the advice, overall its great. Now I've been in the custom audio/video business for 19 years now, primarily residential, so I've formulated some of my own opinions
BIGglaSS offers up some great advice.
Here's the system I'm putting in my '72. I really need to finish wiring it up so I can tell people how it sounds. I'll include retail price to give you an idea on budget. I primarily chose these brands because I use these in marine installs and can get dealer pricing.
Source: Exile Audio Bluetooth Receiver. I had this sitting on the shelf and its quick and easy to install. I may try a RetroSound/Custom AutoSound headunit down the road to see what I think. $50.
Amp: Rockford Fosgate Prime R300x4 50Wx4 RMS 4-Channel Amp. I had the R250X4 in my '67 and it surprised me as far as sound quality on a budget. Again I may upgrade to a nicer one down the road but this will get the system up and running. Plus I like to tinker. $200.
Speakers: (Fronts) Rockford Fosgate Punch P142 4" Coaxial Speakers in the factory dash location. Again had these in my '67 and like them. I made plate out of MDF and a bent a metal bracket to mount in stock location. The dude from Kicker claims these aren't necessary so I plan to try with and without to see what I think. $70.
(Kickpanels). Rockford Fosgate Punch P-1675-S 6.75" Component Set. I've decided to mount the tweeters on the dash where the factory AC vents are. Will play around with locations to get the best sound. $140.
RetroSound Kickpanel pods. I lined the kickpanel and pods with DynaMat I had lying around to help with vibration. $70 + a few dollars for sound deadening.
Subwoofer: Rockford Fosgate PS-8 amplified 8" subwoofer. I've not heard this but will here soon
. Going to place it under the seat. I've set it up to run (2) if I feel the need. This truck still has the gas tank behind the seat so a custom box isn't really an option. $300.
Wire: I've been real happy with T-Spec and since I can get it at cost I'm using it. I'm using 4GA primarily because I can and may upgrade the amp and add another sub down the line. This kit is about $75 but probably could get a good 8-GA with speaker wire for the same price. $75.
Grand total: $905 going off manufacturer's websites.
I'm not saying this an the end-all-be-all system but a good reference point.