To add an amp to the speakers...

Power wire to the battery
Ground Wire to the chassis of the vehicle
Remote wire to the switch power wire on the headunit (red)
RCA cables to the headunit
Speaker wires to the speakers

If doing this, you can skip the speaker wire step when installing the radio. You will instead run speaker wire from the amp directly to the speaker.

8 Gauge Amp Wiring will be fine for an amp running the speakers. However, if you plan to add a sub amp in the future, you will want to use atleast 4 gauge. This prevents you from having to redo your power wires.

As far as the battery, it depends on what battery you are currently. 9 times out of 10, the stock battery will be fine running a speaker amp. However, if it is an older or low cranking amp battery, it will be best to upgrade not only for the audio system, but the car in general.