There are many threads about this and approaches. Do a search in this forum with the keywords 8.4 and amp, I am sure many will come up.
In short, if you do not have an Alpine sub in your truck, you do not have an amp, all of the power is supplied by the radio. To add an amp, you will need a line out converter (can be a simple but high quality one, a JL Fix type device or even DSP) or an amp that accepts high level inputs.
You will need to tap into your existing speaker wires for the signal to the amp. If you don't care about maintaining front to rear fade, you can just tap into the front or rears but if you want to retain the fade feature, you'll need to tap into front and rears.
Do note, your front door and dash speakers are run on the same line, your radio has 4 outputs, not 6. This creates an issue with using existing wiring. While you could just "break" the signal, take it to the amp and then feed it back to the other end of the "break", with the door and dash speakers on the same line it could cause an ohm issue so you are going to want to plan for this. There are many solutions to this, I highly recommend scouring this forum and seeing the different approaches.
Since you already replaced your speakers, my best advice here forward is to make a plan of where you want to be to protect your wallet. I've been down the road in other vehicles, replace the speakers.....sounds good, I want more volume....DAMN! Those speakers won't handle the output of the amp or just sound like crap.....buy a new head unit, doh, now I need to run RCA's and I wasted money on those LOC's.....etc, etc.
