I try to keep the smile on my face while I cringe just a little inside.
I realized recently that with all the different media depictions, misunderstandings, accusations of witchcraft and so fourth that a majority of people don't know what Dungeons and Dragons is exactly, so here is my attempt to set the record straight.
A Different Beast
In most board games, a group of people can sit down one evening, open the box, set up the components, play, then put everything back in the box. Dungeons and Dragons though, is a whole different beast.

Next each player will need to build their character. Some characters can be found online but the point of Dungeons and Dragons is to create a custom character that you want to play, unlike a video game where you slip into Mario or Geralt's shoes.
Lastly and most importantly, you need a dungeon master. This is a person who will run the game. The players each play one character in the world, the dungeon master's role is to play everything else. The creatures, the other characters the players may run into, even the cosmetology the game exists in.
So already you can see that this is levels beyond a game of Monopoly, so this alone makes it all but impossible to start a game on the fly. Usually character creation alone can be at least an hour long process for new players. Some of the more experienced players can streamline the process, but for the uninitiated a lot may need to be explained.
Oh and dice. You need all the dice. Like ever. If you think you have too many dice you don't, you're still short.
The Art Of Roleplay

These are just a few options I could think of on the fly. Honestly a player could do whatever they can imagine to varying degrees of success. The player needs to know that all options are available and what they can and need to do based on who their character is, what their motivation is, what the quest is and where their morals lie. A degree of acting, improvisation and imagination are all required for this exercise, which can be a major departure for most new players who aren't used to getting to be people.
Length and Sessions

Still Want to Play?
This may all seem intimidating, but if you still want to give it a try know that they're are people who are happy to teach newcomers to the game. If we didn't the game would probably be dead. Some Dungeon Masters are even informally tutored by veteran players eager to pass on their skills. THe point is that if you want in you're welcome to join but just know what you're in for.
Also, never split the party.
-JOE