Here's some of the things you need to do:

Genre

You need to decide what type of RPG it is. Is it Fantasy, Sci-Fi, Future, Medieval, or a combination of some of these.

I wrote an RPG called AENAR that is a Fantasy based system.

Races

How many races of people are there in your system? Are there only humans? Or are there a variety of races such as Humans, Elves, and Dwarves?

Aenar has Louki (human), Mujina (dark elf), Chowen (chow dog), Saot (Gerbil), Avian (bird), and Amphibians.

Character Classes

What can the characters in your game be? What careers can they pursue? Every system I have ever played has had: Warriors, Mages, and Rogues (Thief, assassin, bard, etc.).

Skills

An RPG is not complete with a list of things that people do. Characters are rather boring without skills like: Cooking, dancing, juggling, hunting, musical instruments, etc.

Rules

Where would an RPG be without rules? Rules are essential for a successful role playing game. Some rules to consider are:

What constitutes a hit in combat?
How does magic work?
How many hits can a character take before they succum to pain or death?

Dice

Dice are an important part of many RPGs and there are many to choose from: 4, 6, 8, 10, 12, and 20 sided dice are the most common.

Why are dice important? You need a system by which you can determine what happens with characters within the system. Dice are used for determining what happens in combat, magic, tests of the mind, and countless other tests.

Aenar uses 4, 8, 12, and 20 sided dice.

The world

Without a world to play in, an rpg is useless. The world is composed of many parts of what has already been created. If the races in your game are mainly aquatic, your world is not likely to have a lot of dry ground.