1) Select a commander. It must be a legendary creature. The mana used to play this card or activate any of its abilities are the only mana colors you can use in your entire deck.
2) Only one-of-a-kind cards in the deck (except for basic lands).
3) About 46 mana sources (38+ basic lands - which can, of course, be repeated and 8+ other mana sources like artifacts or ramp spells)
4) About 25 - 35 creature cards (again, only use a card once in your deck)
5) 100 card deck (exactly! That includes your commander)
The Basics of the Game:
1) You start with 40 life and your commander in the command zone.
2) Cast your commander whenever but know that if it dies you can redirect it to your command zone. Once in the command zone again, you can recast it for its original casting cost +2 (commander tax) for the number of times it has been cast. For example, if your commander died on the battlefield once and you want to recast it from your command zone it will cost you the original casting cost printed on the card plus two more mana. The next time it dies, it will cost you the original cost printed on the card plus four more mana (2 for the second casting and 2 more for the third casting.) It adds up quickly.
3) Win the game by taking your opponent down to 0 life or by doing 20 damage to the player with your commander only. So if you attack with your commander and cause 21 points of damage over the course of the game, you win the game.
Watch this video as an intro to the commander format.
5 underrated commanders
Watch the video below for help in understanding synergy.