You must have a fairly good idea of what you want before defining your rules. 'Defining your rules' means choosing which attributes your world(s) inhabitants, items, etc will have, which actions they could do, etc.
Example 1. Example
In the StarWars(tm) RPG, rules are based on six face dice (D6). Each player has some skills ( first aid, to pilot ships, to swim, to command, to dodge, to astrogate, languages, blaster, etc), divided in six categories (dexterity, knowledge, mechanics, perception, vigour, technique). They can be sensible to the force or not. They are (or should be) heroes. Game rules also define ships characteristics, E.T. powers, the fight Empire vs Alliance, etc. To determine if an action is a success, dice are rolled and sum should be greater than the difficulty factor. The more the sum is, the more you succeed.
Example 2. Example
In the 'Mage: the Ascension' RPG, rules are based on ten face dice. Each player has three physical attributes (strength, dexterity, vigour), three social attributes (charisma, manipulation, appearance) and three mental attributes (perception, intelligence, cunning). Skills are consciousness, intuition, vigilance, driving, meditation, technology, law, riddles, computer science, sciences, etc. There are nine magics. Entelechy and willpower are really important. To determine if a magic action is a success, dice are rolled and dice greater than the difficulty factor count as successes. The more successes you have, the better you succeed. Dice equal to one are automatic failures and remove one success. If the number of successes is zero, action failed. If the number of successes is negative, action is catastrophic (or worse).
Example 3. Dummy instance
Basic rules could be to use a physical and an intellectual attributes (values from 1 to 6, chosen with 1D6). For physical (respectively intellectual) actions, you roll 1D6 and the result should be greater than the difficulty factor. For fighting (resp. playing chess), each fighter (resp. player) adds 1D6 with its own physical (resp. intellectual) attribute; the greater wins.
After defining your rules, you should know which Properties are needed, which Actions are possible and which Quests are allowed.
You won't have to define non-Action command (i.e. commands acting outside the world, like 'help', 'quit' or 'about'). Metacosm should provide all such commands. So you just have to define Action command (like 'swim', 'kill' or 'steal').