Once you've chosen your Commander, constructed your 100-card Singleton deck, and found a friend or group to play with, you'll need to know the rules. Each Player starts the game with 40 life instead of 20. This lends itself to slightly longer games than typical Magic, especially with four or more players at the table. If a player is dealt 21 points of combat damage by a single Commander, they lose the game. This can help keep games from going on for too long, and requires that you track 'Commander Damage' throughout the game. Poison rules still apply, though in my playgroup we have doubled the number of Poison counters required to kill to follow suit with the increased Life total. If your Commander would be put into the Graveyard or Exile from anywhere, you may choose to instead move it to the Command Zone. This doesn't happen for "return to hand" or shuffle effects, so if your Commander gets shuffled into your Library, say with Grasp of Phantoms and Lantern of Insight, it can be tough to get them back onto the field. Also see Chaos Warp for more Commander shuffling fun. Commanders are prime targets for this and most removal, so it's often advisable to have equipment like Swiftfoot Boots and Lightning Greaves to help protect your Commander once they're on the Battlefield.
As I mentioned earlier, EDH is a very multi-player oriented format, so interactions around the table are highly valued in deck design. The Commander series introduced the Join Forces mechanic, where each player can buy into a certain spell which affects everyone at the table. My personal favorite, Collective Voyage, really gets the game into gear by helping the whole table fetch all the land they need get things moving. There's also the Vow cycle, a cycle of Auras which buff a creature while also preventing it from attacking you or your Planeswalkers, helping to ensure that table politics play an important part of your strategy. They introduced other means of guiding opponents' choices with cards like Edric, Spymaster of Trest which rewards everybody for attacking anybody but you.
That is EDH in a nutshell; interactive multiplayer fun. But there's plenty more to it than I've got space for here. Join me in the comments below to share your own take on the format. What is your favorite Commander? Do you build around a Commander, or do you find a theme first and choose a Commander that fits?
Q: If a creature has both Persist and Undying, will it come back into play after it dies with both a +1/+1 and -1/-1 counter? Wouldn't those counters cancel out, causing the creature to return to play no matter how often it dies?
A: Not exactly, though a creature with both Undying and Persist will be very hard to kill all the same. The abilities will both trigger when the creature dies and, as the controller of the abilities, you'll get to choose in what order they are put on the Stack. You then decide whether they come back the first time with a +1/+1 from Undying, or a -1/-1 from Persist. Once the creature is back on the Battlefield, the other ability resolves from the Stack and does nothing, since the card is no longer in your Graveyard. After that, both abilities can be checked, and it will always come back into play with the opposite type of counter. For a simple example, say you have a Restless Apparition and Mikaeus, the Unhallowed in play. Your Restless Apparition dies, and you choose to have Undying resolve first. Restless Apparition comes back into play with a +1/+1 counter on it. When it dies again, the Undying ability will see the +1/+1 counter and do nothing, but Persist will see that it had no -1/-1 counters, and bring it back into play with a -1/-1 counter. Whenever it dies, as long as it has both Undying and Persist, it will always come back into play with the opposite counter, unless it had both +1/+1 and -1/-1 counters at the time it died.