How do I fill in this background of this public area so there are other people in this world?

[JoshRaed]

Like for example in this scene. What do I put behind the character to communicate an entire cafeteria behind them without crowding them out or adding too much detail? How do I know where these things would be positioned relative to them? How do I make the wall seem far away enough that it looks right?

[kyupol]

Do a simple gradient or a horizontal line.

...and Naruto should use his shadow clones to kick their asses in the next panel

[Lensing]

You need perspective.

You can use a 3d model of a cafeteria and place the objects there.
You can build the background with an app.
Or you can sketch the scene with 1 or 2 point perspective.

I would first draw the room roughly, then know where the ground level is
and the horizon line to be able to place the characters. Then place all
the elements in the scene.

It´s really hard to first draw the characters and then arrange the background around
them. When the perspective is wrong in your picture then the placements of the elements
in the scene will also look wrong.

Some comic and cartoon artists also break the rules of perspective completely, you
have to decide if you want that or if you want to learn correct perspective

[JoshRaed]

For some reason I decided the ground would be so low that you would not see it from this angle when you definitely would.

[JoshRaed]

Is that first part something you genuinely think?

[ThunderChicken]

Simplest way? Make it so that they're sitting at a table near a wall. Draw bricks on the wall and maybe add a poster or two like you'd see in a HS cafeteria. Things have probably changed since I was in HS 3,000 years ago, but we never sat in the middle of the room on purpose. If we could get a table by a wall we were taking it!

The bricks would be easy: Just use a slightly darker shade of the wall colour and draw your lines. Most brick walls inside schools are painted, not left natural, so the mortar lines would be the same colour as the paint. The use of a darker shade of the same colour would simulate the small shadows the mortar would cause. A poster would be simple too. Most posters in a school were made by the students in the school (think upcoming dances, study groups, etc) so make it look like that.

Also, the area behind the table should be a darker colour, probably a grey, because you're looking at a floor here that is shaded by the table. You wouldn't see the wall down there.

[JokesterComics]

Maybe add some windows and make them small to portray that it's further away.

[krazycafetales262]

Add some tables and chairs with people sitting at it. If you don't want to be too detail, do something simple then blur it out. We'll still get the idea without the extra work.

[ThunderChicken]

Just took 5 minutes and did this. You'll excuse the scraggly lines, I'm drawing with a broken elbow, but it shows my point. You can make it a "room" while keeping it simple.

[Calculus_Homework]

Wow! That really came out great. Nice work!!

[ThunderChicken]

Thanks! I tend to go overboard on my backgrounds, but even a simple one can be effective, especially for a cartoony-style comic.

To the OP: If you want to see some examples of how backgrounds don't matter a whole lot, just watch some classic Warner Brothers cartoons, especially from the 50's and 60's. You will see that backgrounds were an afterthought, but it did not detract from the cartoons.

Also, more relevant to your comic example, while you don't need to spend forever on backgrounds you do need to pay attention to the foreground. What are those kids eating in that cafeteria? There is no food on the table!

Here's an example of one of my "went-way-too-far" backgrounds, coincidentally enough, inside a cafeteria. Not a school cafeteria though - this was in a K-Mart cafeteria in the mid 1980's. Ignore the background though, and note that there is food on the table!