Although this is given the GM techniques tag, I'm going to answer it in the context of fantasy adventure gaming.
- You Don't.
The world of the fantasy adventurer is not a world of people as we understand them. Unless you're playing a purely historical game, adventurers are different. Acting like a real person in a fantasy adventure game means A) allying yourself with the most powerful creature you can find, and B) hoping you're not its food. The "real people" as we'd think of them in a fantasy adventure role-playing game aren't the PCs but all the unnamed, disposable extras in the background. They worry about surviving the day, having enough copper pieces to feed their families, and staying out of the way of all the myriad things that can casually, accidentally murder them--which totally includes fantasy adventurers.
In other words, it's pointless to try to get players to have their characters act like real people when real people run away from everything adventurers run toward. Most players will be unhappy playing Papers and Paychecks or Farmer: The Role-playing Game. That's not to say there aren't adventures there, but those are much rarer in the adventure role-playing game genre that's been developed over the past 40 years.
So, while the PCs may not have ever set foot in a dungeon before, adventurers have. Unless your PCs are the universe's first adventurers (which could be an interesting thing, I guess), the how of adventuring is certainly something one would pick up when considering an alternative to the quiet life of peat digger, dirt merchant, or monster servant.
- Bind Them.
The group of random adventurers, while a time-honored staple ("We met at a tavern last night; let's go camping together until we die!") is a pretty shoddy set-up.
Older versions of Dungeons and Dragons postulated the idea of dungeons as similar to Old West Gold Rush towns: While one could gather gold alone, one can gather a lot more gold with more people with strong backs and big biceps to help. And everyone is disposable. The players didn't have to care about their own or other players' characters because nobody named a character until one hit level 3 when he could survive a hit from an orc. Life. Was. Cheap.
Modern role-playing games give starting characters much more depth but sometimes still leave them extraordinary fragile. The fragility of beginning characters might make your players believe them disposable. For the DM, though, they're not. You've a story that you want to tell and a dead character early (when means of return are unavialable) means that story remains sadly untold. And that sucks.
The DM's job, then, is to bind the random PCs, making them care about each other's fates. The easiest way is, of course, forcing a bond upon them: they are all family or are all raised in the same orphanage. They might share culture, education, national or tribal origin, or an enemy. I've gone so far as to impose game mechanical bonds upon player-characters when I knew the players would instantly backstab each other's characters when play began were I to not. (It was a group where oneupsmanship was common in and out of games; there was no way around it.)
You can even leave it up to the players. I explain to the players that the entire world is trying to kill their characters and the only folks their characters can rely on are played by the other people at the table. Then let the players figure out why their PCs are there; if they can't, have them come up with different concepts. Not every concept fits every campaign. For experienced, mature players, that's often enough.
There's no right way to play, and some gaming groups thrive on the conflict that comes from interparty murder sprees and random thievery, but I don't like the game to be about that. I like the PCs fighting the world not each other. If that means imposing cooperation instead of competition at the campaign's start, so be it. The alternative, while it can be entertaining and amusing, is chaos.
