firstly head unit change should be simple take out and re-fit new 1. may need car scarts , this allows you to fit the stereo to inner wiring with no cutting wires.
speakers should be just a straight swap if they are the right size in the front.
the rear you will need to run througn behind the dash and under door sills into rear door cards to where the speakers are fitted, depends on 3dr or 5 dr.
amp and sub. dont buy the cheapest lead you find (mains). buy what is more powerful than the amp, this way it wont burn out on use. This is red, comes with in-line fuse. connect to live on battery and run into car and through into the boot where you can screw it into the live position marked with a +
then you'll have a short black wire which earths to the car, screw to the - on the amp, take bolt out of the boot shut clamp and wrap wire round bolt and rescrew it in. (this is where i have mine).
Then you'll have a blue wire (REMOTE). It goes into the other socket on the amp and connects to the remote which is blue on the back of the stereo, may need to cut a break into the remote from stereo.
basically the blue wire just turns the amp off on/off with the stereo, this way your battery dont go dead.
They are pretty simple really if you you can put your mind to it.
Mine took longer to get the wire from battery into the car itself than it did to wire it in.
Any more help needed then let me know.
I'll be as helpful and simple termed as i can.
A place like halfords will rip you off to install this, they will prob want a few hundred for fitting only and it will take you about 2 hours max really if everything runs smoothly