A ground plane is a large area of copper in a printed circuit board design that is connected to ground on the power supply.
You can try to find existing footprints for components, but most components will require custom footprints.
A Gerber file (also known as artwork) is a 2-D graphical representation of a single layer of a PCB. A typical design will have individual Gerber files for each layer (e.g., top copper, bottom copper, top silkscreen, bottom silkscreen, top soldermask, bottom soldermask) of a PCB.
Solder mask is the thin polymer layer that is applied to a printed circuit board to insulate copper traces from unwanted connections. It is often green, red, or blue, and is put over all parts of a PCB except where components are to be soldered.
While Cadence does have some built-in footprint libraries, they are limited due to the hundreds of thousands of different parts in existence. Additionally, most companies make their own footprints to match the specific manufacturing processes that they use.
See the What is a Printed Circuit Board? page for more information about PCBs and the general design flow for creating a PCB.
This walkthrough goes through the process of creating a PSoC® 4100S Plus prototyping board PCB footprint.