format PE64 GUI 4.0
entry Start

include 'win64axp.inc'
;include 'encoding\WIN1251.INC'

section '.code' code readable executable

Start:  push rbp
         sub rsp,16

        invoke  GetModuleHandle,0
           mov  [wc.hInstance],rax
           mov  [hInstance],rax
        invoke  LoadIcon,rax,17
           mov  [wc.hIcon],rax
        invoke  LoadCursor,0,IDC_ARROW
           mov  [wc.hCursor],rax
        invoke  RegisterClass,wc
          test  rax,rax
            jz  Error
        invoke  LoadMenu,[wc.hInstance],37
        invoke  CreateWindowEx,0,w_class,w_title,WS_VISIBLE+WS_OVERLAPPEDWINDOW,0,0,100,100,NULL,rax,[wc.hInstance],NULL
           mov  [hMainWindow],rax
          test  rax,rax
            jz  Error

proc WindowProc hwnd,wmsg,wparam,lparam
;                 WTF???
;          What I must write here???
;              Please, help!!!


Error:
        invoke MessageBox,NULL,w_error,NULL,MB_ICONERROR+MB_OK
section '.idata' import data readable writeable

     library kernel32,'KERNEL32.DLL',\
         user32,'USER32.DLL',\
         gdi32,'GDI32.DLL',\
         comctl,'COMCTL32.DLL',\
         comdlg32,'COMDLG32.DLL',\
         msvcrt,'MSVCRT.DLL'

     include 'api/kernel32.inc'
     include 'api/user32.inc'
     include 'api/gdi32.inc'

 import msvcrt,\
        sprintf,'sprintf'

 import comctl,\
        CreateStatusWindow,'CreateStatusWindowA',\
        InitCommonControlsEx, 'InitCommonControlsEx'

 import comdlg32,\
        GetOpenFileNameA,'GetOpenFileNameA',\
        GetSaveFileNameA,'GetSaveFileNameA',\
        ChooseColor,'ChooseColorA'


section '.data' data readable writeable
   wc WNDCLASS  0,WindowProc,0,0,NULL,NULL,NULL,COLOR_BTNFACE+1,NULL,_class

align 1
   INIFileString rb 1024
   OpenFileString rb 1024

     w_title TCHAR 'Mandel Machine Movie Maker'
     w_class TCHAR 'MMMM'
     w_error TCHAR 'LOL, Startup failed =)'
align 8
  hInstance             dq ?
  hMainWindow           dq ?
section '.rsrc' resource data readable

     menu main_menu
     menuitem '&File',10,MFR_POPUP
     menuitem '&Open',11,IDM_LOAD    

What should I do with WindowProc?

format PE64 GUI 5.0
entry start

include 'win64a.inc'

PROGRAM_WIDTH = 1280
PROGRAM_HEIGHT = 720

section '.text' code readable executable

  start:
        sub     rsp,8
        invoke  GetModuleHandle,0
        mov     [wc.hInstance],rax
        invoke  LoadIcon,0,IDI_APPLICATION
        mov     [wc.hIcon],rax
        mov     [wc.hIconSm],rax
        invoke  LoadCursor,0,IDC_ARROW
        mov     [wc.hCursor],rax
        invoke  RegisterClassEx,wc
        test    rax,rax
        jz      error

        invoke  CreateWindowEx,0,_class,_title,WS_VISIBLE+WS_DLGFRAME+WS_SYSMENU,128,128,PROGRAM_WIDTH,PROGRAM_HEIGHT,NULL,NULL,[wc.hInstance],NULL
        test    rax,rax
        jz      error

  msg_loop:
        invoke  GetMessage,msg,NULL,0,0
        cmp     eax,1
        jb      end_loop
        jne     msg_loop
        invoke  TranslateMessage,msg
        invoke  DispatchMessage,msg
        jmp     msg_loop

  error:
        invoke  MessageBox,NULL,_error,NULL,MB_ICONERROR+MB_OK

  end_loop:
        invoke  ExitProcess,[msg.wParam]

proc WindowProc uses rbx rsi rdi, hwnd,wmsg,wparam,lparam

; Note that first four parameters are passed in registers,
; while names given in the declaration of procedure refer to the stack
; space reserved for them - you may store them there to be later accessible
; if the contents of registers gets destroyed. This may look like:
;       mov     [hwnd],rcx
;       mov     [wmsg],edx
;       mov     [wparam],r8
;       mov     [lparam],r9

        cmp     edx,WM_DESTROY
        je      .wmdestroy
  .defwndproc:
        invoke  DefWindowProc,rcx,rdx,r8,r9
        jmp     .finish
  .wmdestroy:
        invoke  PostQuitMessage,0
        xor     eax,eax
  .finish:
        ret

endp

section '.data' data readable writeable

  _title TCHAR 'Mandel Machine Movie Maker',0
  _class TCHAR 'MMMM',0
  _error TCHAR 'Startup failed. Hahahahaha',0

  wc WNDCLASSEX sizeof.WNDCLASSEX,0,WindowProc,0,0,NULL,NULL,NULL,COLOR_BTNFACE+1,NULL,_class,NULL

  msg MSG

section '.idata' import data readable writeable

  library kernel32,'KERNEL32.DLL',\
          user32,'USER32.DLL'

  include 'api\kernel32.inc'
  include 'api\user32.inc'

section '.rsrc' resource data readable


  menu main_menu
       menuitem '&File',0,MFR_POPUP
                menuitem '&Load...',10
                menuitem '&Close',11
                menuseparator
                menuitem 'E&xit',1,MFR_END
       menuitem '&Help',0,MFR_POPUP + MFR_END
                menuitem '&ABOUT...',12,MFR_END
    

Go learn the Win32 API from TheForger, then come back to FASM later...
http://www.winprog.org/tutorial/

  .openseq:
        mov     dword[of.lStructSize],sizeof.OPENFILENAME
        mov     dword[of.lpstrFile],OFN_ALLOWMULTISELECT ;that's is not correctly
        mov     dword[of.lpstrFile],OpenFileString
        mov     dword[of.nMaxFile],1024
        invoke  GetOpenFileNameA,of
        test    rax,rax
        jz      .returnz
        invoke  CreateFileA,OpenFileString,GENERIC_READ,0,0,OPEN_EXISTING,0,0
        mov     r15,rax
        test    rax,rax
        jz      .returnz