public OnPlayerConnect(playerid)
{
  ShowPlayerDialog(playerid, 1, DIALOG_STYLE_LIST, "Team Choosing", "Please choose your team :\nRed Team\nBlue Team", "Select", "Cancel");
  return 1;
}

public OnDialogResponse(playerid, dialogid, response, listitem, inputtext[])
{
   if(dialogid == 1 && response)
   {
      switch(listitem) 
      {
         case 0:{ }
         case 1:
         {
              // do somethings here for the Red Team
         }
         case 2:
         {
              // do somethings here for the Blue Team
         }
     }
  }
  return 1;
}