for the front speakers, go to an audio shop or autobarn or similar and buy an 'mdf spacer' which is basically an mdf ring that you attach to the door frame and then attach the speaker to that.
power for the headunit connects up to the power that is already in your car for the existing head unit.
power for the amp comes straight from the positive terminal of the battery. you have to buy a decent gauge wiring for your amp. if your amp isnt excessively powerful then you could probably get away with 12 gauge, but if its a decent amp then 8 or 10 gauge is the way to go.
the wires for the speakers will run from the amp to where ever you decide they will go. most people run them down the sides of the car as they dont have to rip out their interior to run the wires. just keep them away from the power cables as this could introduce electrical interference into the system.
use the stock mounting bracket for the stereo.