How do I receive input from the user?

.data 
StringOriginal: .asciiz "Frase Introduzida:\n"              #ex_msg_og
StringFinal: .asciiz "\nFrase Encriptada:\n"
String: .asciiz "Hello World"

.text 
Main:
    la $v1,String #load addr of string into $v0
    la $t1,($v1)  #copy addr into $t0 for later access
    lb $a1,($v1)  #load byte from string addr
Loop1Original:  #strlen_loop
    beqz $a1,Memoria                        #alloc_mem  
    addi $a0,$a0,1 #increment strlen_counter
    addi $v1,$v1,1 #increment ptr
    lb $a1,($v1)   #load the byte 
    j Loop1Original

Memoria:
    li $v0, 9 #alloc memory, $a0 is arg for how many bytes to allocate
             #result is stored in $v0
    syscall
    la $t0, ($v0) #$v0 is static, $t0 is the moving ptr
    la $v1, ($t1) #get a copy we can increment

    add $t1, $t1, $a0 #add strlen to our original, static addr to equal last char
    subi $t1, $t1, 1  #previous operation is on NULL byte, i.e. off-by-one error.
                    #this corrects.
copy_str:
    lb $a1, ($t1) #copy first byte from source

Loop2Original:
    beq $a0,0,Final 
    sb $a1,($t0)            #store the byte at the target pointer
    addi $t0,$t0,1          #increment target ptr
    subi $t1,$t1,1
    subi $a0,$a0,1
    lb $a1,($t1)            #load next byte from source ptr
    j Loop2Original

Final:
    la $a1,($v0) #store our string at $v0 so it doesn't get overwritten
    li $v0,4 #set syscall to PRINT

    la $a0,StringOriginal  #PRINT("original string:")
    syscall

    la $a0,($v1)      #PRINT(original string)
    syscall

    la $a0,StringFinal #PRINT("copied string:")
    syscall

    la $a0,($a1)      #PRINT(strcopy)
    syscall

    li $v0,10         #EXIT(0)
    syscall