So let's talk about making the switch to Competitive Multiplayer EDH. First of all, if you're coming from Casual/Regular EDH you should know there's a whole lot less confusion and everything is a lot more clear. There is no ridiculous house rules, silly political alliances, no need to worry about keeping up the with the conversation, no need to worry about unfair advantages, and you don't have to worry about whether you're playing any of those things that players "hate" like mass land destruction, instant wins and infinite combos. Your goal is just to win! You don't have to worry about building a unfun, or unfair deck. You don't have to worry about being on theme, or making sure your cards aren't too powerful! You don't have to ask before every game "Is it okay if my deck has a combo?" and all of those questions. SCREW all of that! You can build what you want as long as it's LEGAL, make it as STRONG as you desire, and as BRUTAL as you like. When you get to the game your goal is going to be to WIN. If Casual EDH was better than Competitive Multiplayer EDH, I'd be playing Casual EDH but noooo I like my powerful deadly EDH decks.
If you're coming from other formats like Duel Commander, Standard, Modern, Legacy or Vintage. You know this! You're about to enter the world of multiplayer, you will have multiple opponents, more threats, and more problems. Plus it will be more fun, and more rewarding when you beat them all. You won't get trophies like they do at the Standard Pro Tour but its like beating 3 chess players with one chess game. But besides that the rewards for winning at cEDH Multiplayer is steadily increasing. There's events, local tournaments and large online tournaments.
So its really easy if you've already learned rules for MTG in general, and you want to start with the basic EDH rules on the mtgcommander.net/ website.
First of all choose a powerful combo, or a powerful top tier commander.
Secondly you'll need Gatherer which works best for me its simple and lays out all the cards just fine, a couple of other deck lists from mtgs.com or cEDH Reddit using the combo or commander you chose, and https://edhrec.com/
Third, just pick out staples, the core cards, strong cards, and stick with a list that isn't too far from what the best known lists are. You need to keep in mind that you will have more than one opponent, your strategy, win condition, and the cards you add to the deck need to reflect that.
When you play in multiplayer games you need a combo or win condition that can kill all of your opponents. Otherwise you will leave one alive and that one would possibly beat you. Some examples are storm combo using Aetherflux Reservoir (with a high storm count to kill everyone off), instant wins like laboratory maniac, flash/protean hulk combo, ooze combo (infinite damage or high levels of damage) and even milling all of your opponents out or making them draw everything in their decks.
Finally, you're ready to go play some cEDH just hit up shops that have a competitive group, find competitive players on cEDH Reddit or MTGS.com or join the cEDH Reddit Discord Server Channel. You can play both online or offline with paper cards (which can get expensive) be prepared to spend a couple of thousand dollars on a cEDH deck in paper. For online play you can use MTGO, Lackey, XMage, and the best of them all is Cockatrice!
Alright I'm going to wrap this up by letting you know that I'll be doing many courses on deck building to help you create competitively viable, winning and powerful EDH decks.