If you mean saving a program itself...
While holding L or R, press SAVE on the top portion of the touch screen. You will be taken to a dialog to save your program.
If you meant saving data for things such as player information and progress, please reply accordingly so I can tell you.
Everyone must start off somewhere. Don't worry.
An entity can mean literally anything. In your game, it can be a wall, an enemy, a laser, or even the player itself.
You can represent these as a number in a 2-dimensional plane, assigning a number for each different type of entity.
E.g.: 0 can be nothing, 1 can be a wall, 2 can be an enemy, etc.
You say that as if the exploit was a good thing. (It took SmileBASIC off the eShop!) Please remain a lawful member of the Nintendo community and don't hack your systems! c:
Well, you must start off by understanding the concept of an entity map. These are extremely useful when programming non-text console games. It usually is a 2-dimensional numerical array the size of the console screen, and will hold basic data of the entities on screen.
Perhaps you'd be able to do create a similar game using only the console, but you need to learn how to handle entity maps and objects first.
As a start, you should have a little character that moves with the D-pad on the console screen. Try to program it by yourself before asking for help.
The correct series would be Wolfenstein, right? As long as the published release does not include any copyrighted assets, such as visual elements and official level layouts, it should be fine.
Retro City Rampage is a great GTA-style game for 3DS. You can find it on the eShop.
If you want a Garry's Mod-style game, you're better off just playing the real thing.