Step 1: Gather some friends. If you do not have a long list of friends who are interested in D&D (or TTRPG’s in general), look to your local game shops, or your social media gathering point of choice (Facebook, Myspace etc). There are endless lists of LFG (Looking For Group) pages and societies where a player can make new friends with the click of a few buttons.
Step 2: Gain access to the rules. Do this by purchasing the rule books from participating stores, or online.* Additionally, make sure everyone has access to a full set of standard gaming dice, which should include seven separate dice of various sizes; four sided (d4), six sided (d6), eight sided (d8), two ten sided dice to denote tens and individual numbers (otherwise known as percentiles, or d10’s), twelve sided (d12), and most importantly a twenty sided dice (d20). Your d20 is the dice most often used in D&D, for everything from climbing a tree to swinging your sword.
Step 3: Read the books. No, seriously, read them, all of the players, all of the information you can possibly get. READ. Learning the rules helps everyone in the game, speeding up the flow of actual gameplay and keeps everyone on a level playing field.
Step 4: Play! The opening pages of the Player’s Handbook (PHB) and the Dungeon Master’s Guide (DMG) explain the basic gameplay loop of D&D, so start there and work forward.
But what does any of that actually mean?
With your group, determine one person to be the “Dungeon Master”. Before we go further, let’s clear up a misconception: THE DUNGEON MASTER IS ONE OF THE PLAYERS. The dungeon master masters the dungeon. They act as a referee in a sense, giving context for the world, along with controlling the monsters and dangers within it. HOWEVER. This does not put the DM and the players at odds. The players and DM are, in D&D, a cooperative unit. D&D is about telling a collaborative story between the DM and players. The DM should not be going out of their way to kill the other players (it’s not exactly a fair fight when a person can just say “A giant dragon appears and kills you, the end,”); nor are the players going to “defeat” the DM. The adventurers can kill the Lich, whose actions are controlled by the DM, but this is not the same as fighting the DM directly, and should never be confused for such.
With that out of the way, your group can focus on how they want to play. The theme or genre of your groups’ game is something not to be taken lightly. Not everyone wants to play the standard high-fantasy light v. dark campaign as made popular by the likes of Tolkien. The DMG goes into further detail here (seriously, most of what I’m saying here is in the DMG, go read it!), so start there. Maybe your group is made up of Conan fans, hungry to explore a dark and violent world, full of desolate wastelands and debauched settlements hiding from the evils outside. Perhaps your group wants to take a political approach and become involved with the machinations of the local nobility, rather than hunting monsters in the traditional sense. Or heck, maybe your group wants to stage highway robberies on lightning-trains guarded by dinosaurs (looking at you, Eberron!). Make sure your group communicates openly about what each person wants to get out of the game, to make sure everyone has a good time.
Several complete adventures have been released by Wizards of the Coast for players to use, starting with the Rise of Tiamat all the way to the upcoming Rime of the Frost Maiden. Even more ready-to-be-played adventures are available online (check out the Dungeon Master’s Guild website, dmsguild.com , for an excellent resource full of player-created material , and help support content creators!), so your group isn’t going to starve for adventures.
If your group can't find anything that offers quite what they're looking to play, creating your own adventures (and even setting) can be an exciting exercise in player cooperation. Your standard D&D game needs nothing more complex than "Several people want to achieve a goal". The people may be strangers, side-eyeing each other over their mugs of ale and waiting to see who can be trusted, or they could be old friends gathering for one last score. The goal itself might involve a local goblin sighting, or stealing an ancient treasure from a rediscovered tomb. Don’t get too bogged down in the minutiae of the details; as long as your party has a defined goal, the rest can come later. The specific details (NPC’s, towns, governments, and other various worldly concerns) are something for the DM and players to work out together over the course of the game.
And that's it! Your players are gathered, your setting and adventure are decided. With the rules at hand for easy reference, your party can set about creating characters (which can be an incredibly fun activity), and the adventure can finally begin. Good luck to you, stalwart adventurers!
*What you ABSOLUTELY CANNOT DO, UNDER ANY CIRCUMSTANCES, is find free and readily available PDFs or photocopies online, which are free and extremely easy to find, thus bypassing the somewhat exorbitant costs of the individual rule books that can act as a barrier to new players looking to explore a hobby without making significant financial investments. It’s not like the internet exists as a tool to share ideas and information between people across the world, after all.**
**Extreme sarcasm is extreme.
***Some of the existing printed material available for purchase is out-of-date and no longer represents the current rules of D&D 5th edition. The "Starter Pack", for example, is almost completely incompatible with the rules put forth by the PHB, DMG and Monster Manual.