I am about to teach a group of five beginners how to play D&D. None of them know how the game works, but they have heard good things about it and would like to try it. What is the first thing I should teach them, aside from character creation? How should I go about introducing them to roleplaying and playing in general?

All advice should be filtered through your knowledge of the players. (The approach is likely different if they're board gamers than it is if they're improv actors.)

I'm sure somebody is going to suggest showing them Critical Role or some similar thing, but I'd advise against it, as it could create expectations that they (and you) can't live up to.

The very first thing is not character creation, but the concept of an RPG. Not just the "you will make and play a character, while I play the entire rest of the world", but also the fact that it's a cooperative game, and that the characters they play should have reasons to work together and to seek out adventure.

Then give them a very simple setup: the world (presumably fairly generic fantasy), the initial conflict (make sure it's not one that forces them to be immediately proactive), and also the tone you're going for (this last one should be at least somewhat negotiated with the players)

Then it's time for character creation, ideally in a group setting. It's more important to get their concepts locked in, rather than the mechanics.  Encourage them to form connections between their characters. Let them introduce things into the setting if their character concepts call for them. Keep the amount of choices limited at this point. (For instance, they're unlikely to need to know the difference between wizards, sorcerers, and warlocks yet. You also don't need to put all the PHB races on the table.) Once the general ideas are settled, then walk them through the mechanical part. Assure them that they'll be able to go back and make changes to their choices once they get a better feel for things.

In the first play session, I suggest starting them off with an early combat. The first fight is going to take a while. Make sure there's more going on than a straight "line up and beat on each other". Have the enemies use tactics, use the environment against the players, etc., to establish the idea early that this is something the players can do. Since the players don't know yet what they're expected to do, they're going to try all sorts of things, and you're going to have to roll with it.

You're also going to want to introduce the mechanics of social interaction and general "using skills to solve a problem" early on, but give them a chance to digest between each.

If they really know nothing, I might not even do character creation. Just give them pregens. They wont understand what’s happening anyway, or why a 16 is so much better than a 15, or why most everyone can get away with being really dumb or really weak. So if walk them through assigning ability scores, they’ll just end up putting the scores where you tell them to. Cut out the middle man, and just make some characters. Maybe let the players name them, but that’s about all.

Then you play a couple sessions, to let them understand how things work — it’s much easier to understand the value of your initiative roll, or proficiency in deception, after you’ve used it a few times. 
Then after they’ve played a little, if they’re still interested, you talk about starting a “real” campaign and then rolling up their own characters.

And before you start give them homework of reading the PHB chapters 7,8 and 9.

The first couple of sessions should probably be throw away sessions meaning, give them some pregenerated characters, maybe use an adventure from a box set and take it slow. After they start to get the feeling for it then let them make characters. It can be overwhelming at first but breaking down that challenges will usually come down to a dice roll v. a difficulty should alleviate a lot of anxiety. Also, tell them there are not too many wrong moves and the goal is to have fun, however also tell them that if you ask them a couple of times if they are sure they want to do something, they may want to rethink their choices hahahaha.

Quote from Xalthu >>

If they really know nothing, I might not even do character creation. Just give them pregens. They wont understand what’s happening anyway, or why a 16 is so much better than a 15, or why most everyone can get away with being really dumb or really weak. So if walk them through assigning ability scores, they’ll just end up putting the scores where you tell them to. Cut out the middle man, and just make some characters. Maybe let the players name them, but that’s about all.

Then you play a couple sessions, to let them understand how things work — it’s much easier to understand the value of your initiative roll, or proficiency in deception, after you’ve used it a few times. 
Then after they’ve played a little, if they’re still interested, you talk about starting a “real” campaign and then rolling up their own characters.

And before you start give them homework of reading the PHB chapters 7,8 and 9.

 Yeah I would either do pre-gens OR have the first session be just character creation, so everyone can do it together and you can address questions when they come up. 

For sure start at level 1 too. I think of of the worst things you can do for new players is starting them at level 3. Sure that's when a lot of abilities come online, but levels 1 and 2 cover a lot of baseline stuff that players will need time to get to know, and once you feel they're getting the hang of one concept, then you can advance their level and move onto the more in-depth stuff. 

I might also consider reading the "What is D&D" bit from the start of the players handbook together and starting the conversation with that. 

I would start at the beginning.  Typically I suggest using DND Beyond for Character creation, but in essence that does not benefit the new player. 

Come prepared with a collection of blank character sheets. If they don't have a PHB I would also suggest creating some handouts.  Explaining different things.  And if you can hand them out a few days in advance.

Explain what the different stats are and what they do.

Keep the first time classes to the basics and avoid Sorcerer and Warlock.

Explain what each of the classes do.  

For the first character I may suggest doing a dice roll.  I like 4d6 select the best 3.  I often allow a reroll of 1 or 2 or if the total on 3 is 10 or less.  This way they will be a little good at everything.

Show on your handouts what stats are important to the various classes. 

Only use the basic races species.  I.e. players handbook only, i.e. elf, dwarf, human, etc. and the benefits of each race species . 

Build their first character using PHB and at level one.  Level one is very good so they can learn and grow into a more complex character. 

I agree with the above. 

I have introduced several new players to D&D in the last few years, from age 6-45.  I strongly recommend a discussion beforehand of what to expect and how the game is played.  Start with an email or text and then reinforce the same ideas when you sit down.  Furthermore, I recommend very generic characters.  I play 2nd edition rules and for the first dungeon I make all the characters a level 1 fighter with identical gear (one sword, chainmail and a shield.  No missile weapons.  No races.  No magic items).  Furthermore, they have no stats besides and AC, HP and THACO.  No background story.  Just the most basic information to play the game.  Let's face it....rolling characaters and selecting gear is boring, even with experienced players and takes 30 minutes under the best of circumstances.

I then run them through "A Fork in the Road."  I got this adventure from a website for free (maybe this one???).  It is a PERFECT first dungeon.  It runs 45-60 minutes of real time.  It has decision making, roleplaying, non-combat encounter, and several fights.  This can be a stand-alone dungeon but it can also easily lead into a campaign as well.  I will post it below; it is only one page.  

One caveat, both the times I ran it the players wanted more after the last bar fight.  This is a good problem (they were having fun and did not want it to end) and a bad problem-I had nothing else on hand except for a massive campaign that could not piggy-back onto this adventure.  I ended up making up some encounters (will-of-the-whisp at a swampy part of the river, a quest to kill a giant reptilian animal in the river for leather to make exotic hide armor and a riddle with a fairy dragon as a special one-off for a player who loves dragons of all sorts).  The encounters went pretty well considering they were being created as we went but I would suggest having 3-4 in your back pocket in case your players want more game time.

If people like D&D, and you want to make this a campaign, you can then flesh out the characters, giving them stats, names, etc.  I would also let them pick a class and race and start at 2nd level.  Rules are made to be broken after all.

A Fork in the Road

Background

You are amateur adventurers that have all been hired by a town merchant, Hudson.  He is transporting potion ingredients from his town of Firebug to Fort Muck, a local trading post.  The trail between Firebug and Fort Muck passes through a hostile wilderness, and he is afraid of being robbed by bandits. In the past, he has been forced to pay “tolls” or even run for his life.  In exchange for protecting him, he has offered 20% of his profits.

While travelling along to Fort Muck, you come to a river.  The river has a high side and a low side.  You know you must follow the river to reach Fort Muck but you can either walk along the ridge or walk along the riverbank. Hudson has taken this route before and he is concerned.  Although he says the ridge is the easier route, bandits usually guard it.  However, recent heavy rains may have flooded the river, filling the canyon with water.  Walking in the river valley is usually a more difficult passage; there are swampy areas.

The High Road

The high road winds along the side of the riverbank.  Walking is easy along the broad trail. You come to a narrow rope bridge, spanning across the river.  A swarthy looking man watches from a perch on a rocky outcropping. What do you do?  The player characters must sneak past him, escape into the bushes or find another way to deal with the situation.  If indecisive, 2 bandit appears, and bluffs, “We have a dozen archers with arrows at the ready.  Hand over the gold!”

The Low Road

Walking is easy for some time, as the river cheerfully chatters across the rocks and boulders.  Hudson thinks you are about halfway there.  However, as you round the bend, you can see that the river has flooded this area.  The canyon is deep on both sides, nearly sheer.  The river floods the entire canyon, with no dry land on either side.  The water seems shallow, and there are a few boulders sticking out of the water.  It seems that about 50 yards upstream, the river runs through a very narrow shoot, from the sound of the rapids.  There looks to be dry areas on either side of the river near the shoot. What do you do?  They can: swim across the pool, leap across the stones, look for an easier crossing or invent some other way to cross.  After passing the shoot, they see a bridge overhead, spanning the canyon.

The Negotiation

Upon reaching Fort Muck, Hudson takes the players to a seedy bar where the trade is scheduled to take place.  The small building is packed with dozens of patrons.  It mostly consists of humans, although there is a group of dwarves in the corner, keeping to themselves.  The rowdiest group is at the bar, singing lewd songs.  Other groups are talking loudly.  It is a cacophony.  Smoke from a boar roasting over the fire fills the air.  After a bit, Hudson calls the players over – he is having trouble negotiating with the buyer, a man nicknamed Gavin Stonepaw.  Gavin has a red cloak, trimmed in shimmering silver.  Strange golden googles rest upon his forehead.  Hudson wants a better price than 10 gold per potion ingredient and would like their help. A better price means more pay for the players.  Negotiate, intimidate or accept the deal.  20 potion ingredients total. 

Bar-Room Brawl!

After the negotiation is complete Hudson pays each player their wages. He thanks them for their services and then leaves the tavern.  At the same time, a group of four drunk bar patrons stand up and approach.  It is the same men who were singing loudly at the bar.  They saw the money and try to threaten everyone into handing it over. Several have picked up bar stools and one in front has drawn a short sword.  They are all large men.  But wait, the one with the sword has green skin and pointed ears; he is no man at all!  He angrily brandishes the sword!  “Give us all of the gold”, he bellows. Fight or hand over the money.