HOW-TO Swap some characters

Hello! I'm trying to swap some characters in this form, in Linux+NASM:
abc -> cab -> bca -> abc

I tried the following method

section .data
	string: db 'abc',10
	strlen:  equ $-string
section .text
	global _start
_start:
	mov ebx,1
	add ebx,string
	mov string, [ebx]

But it gives me these errors:
error: invalid combination of opcode and operands

Please help me understanding what it means and how to do it in the best way.

mov ebx,1
	add ebx,string
	mov string, [ebx]

On the third line you're trying to copy the second character of string into the address of the first character in string. I don't think that's your actual intention though.

that code is all wrong. I have solved the problem already :

section .bss
	swap resb 3 
   %macro print 2 
      mov   eax, 4
      mov   ebx, 1
      mov   ecx, %1
      mov   edx, %2
      int   80h
   %endmacro

	%macro toswap 1 
      mov   eax, [%1]
	  mov	[swap],eax
      mov   eax, [%1+1]
	  mov	[swap+1],eax
      mov   eax, [%1+2]
	  mov	[swap+2],eax
   %endmacro

section .data
	alfa  db 'abc'
	alfalen equ $-alfa

section .text
	global _start
_start:
	mov si,3
	mov bp,1
	print alfa, alfalen
.loop:
	
	toswap alfa

	mov ah, [swap]
	mov bx, [swap+1]
	mov al, [swap+2]
	mov [alfa],al
	mov [alfa+1],ah
	mov [alfa+2],bx
	inc bp
	print alfa, alfalen

	test bp,si
	jnz .loop
	
;***EXIT***
	mov eax,1            ; (sys_exit)
	mov ebx,0            ; Exit with return code of 0 (no error)
	int 80h

Links that helped me

Linux Assembly - http://www.linuxassembly.org
NASM Manual ( available in doc/html directory of source )
Assembly Programming Journal - http://asmjournal.freeservers.com/
Mammon_'s textbase - http://www.eccentrica.org/Mammon/sprawl/textbase.html
Art Of Assembly - http://webster.cs.ucr.edu/Page_asm/ArtOfAsm.html
Sandpile - http://www.sandpile.org
comp.lang.asm.x86
NASM - http://www.cryogen.com/Nasm
Asmutils-HOWTO - doc/ directory of asmutils

http://maven.smith.edu/~thiebaut/ArtOfAssembly/artofasm.html
http://www.ibm.com/developerworks/library/l-gas-nasm.html ///very helpful
http://maven.smith.edu/~thiebaut/ArtOfAssembly/artofasm.html
http://www.laynetworks.com/assembly%20tutorials3.htm
http://www.nasm.us/

Be blessed