Q. Show a string in assembly language?
MOV AH, 09H
MOV DX, OFFSET   BUFF
INT    21H
Here data in input buffer stored in data segment is intended to be displayed on monitor.
A complete program:  
Input a letter from keyboard and respond.  "The letter you typed is ___".
CODE SEGMENT 
; set the DS register
                        MOV AX, DATA
                        MOV DS, AX
; Read Keyboard
                        MOV AH, 08H
                        INT   21H
; Save input
                        MOV BL, AL
; Display first part of Message
                        MOV AH, 09H
                        MOV DX, OFFSET MESSAGE
                        INT 21 H
; Display character of BL register
                        MOV AH, 02H
                        MOV DL, BL
                        INT 21 H
; Exit to DOS
                        MOV AX, 4C00H
                        INT 21H
CODE ENDS
DATA SEGMENT
MESSAGE DB "The letter you typed is $"
DATA ENDS
END.