You can probably get a copy of the game off on the Internet, if you know where to look. Good luck editing it though.
I'm sorry but you won't find the source code for Animal Crossing, or any other console game out there, mainly because 1) It's distribution would be incredibly illegal, and 2) Companies tend to keep that kind of thing under lock and key (Valve excluded).
Even if you could decompile it, you would get stuck with (most likely) a bunch of assembly code. For example, see the extremely simple "Hello world!" program done in C below.
CODE
#include <stdio.h>
#include <stdlib.h>
int main()
{
puts("Hello world!");
return EXIT_SUCCESS;
}
Incredibly simple right? Now let's compile it into assembly code.
CODE
.file "hello.c"
.section .rodata
.LC0:
.string "Hello world!"
.text
.globl main
.type main, @function
main:
leal 4(%esp), %ecx
andl $-16, %esp
pushl -4(%ecx)
pushl %ebp
movl %esp, %ebp
pushl %ecx
subl $4, %esp
movl $.LC0, (%esp)
call puts
movl $0, %eax
addl $4, %esp
popl %ecx
popl %ebp
leal -4(%ecx), %esp
ret
.size main, .-main
.ident "GCC: (GNU) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)"
.section .note.GNU-stack,"",@progbits
Now imagine several hundred thousand more lines of that, without any documentation.
Animal Crossing is actually an incredibly complex game. Just because it doesn't look like there's much going on behind the scenes doesn't mean that's the truth. If you want some advice, learn how to program. Animal Crossing is essentially a 2d game, and there are plenty of great 2d drawing libraries for almost any programming language.