Typical amplifier wiring includes a proper sized main positive cable that is fused near the battery, a proper size ground, and a "remote" signal wire from the head unit or other source.

Size or AWG of wiring,   
Typically a positive from the battery must be fused as close to the battery as possible. The standard for determining size depends on the length required, and the maximum rated output of the amplifier. A rule typically used for the wire AWG-size is 2 sizes per 100 watts==

100 watt rms amp needs 12 gauge (awg) while a 300 watt rms amp requires an 8 gauge wire.

Ground wires should to be the same gauge as power, but 2 sizes smaller is acceptable. They need to be as short as possible, and tapped into the body somewhere close to the amp

The remote wire  is normally a 12 gauge, sourced from the auto antenna lead at the head unit.