Are you talking Mainframe or PC assembler?
How many assembler programs have you written?
YOU LUCKY DOG!!

Assembler was and is my all time favorite language to code in!!
Basically, you might have something like this (part code, part psuedo-code)
db user_prompt "Please Enter Your data 0x10 0x13";
db user_buffer 100; -- not sure on that syntax
// Use DOS interupt xx to write message to screen
move message to appropriate register
move 10h to ax; -- or whatever function the display to screen uses
int 21h -- call the interupt to display message
// Use DOS interupt xx to read data from keyboard (STDIO <Standard Input/Output)
move message to appropriate register
move 11h to ax; -- or whatever function to read from keyboard
you'll have to find which register to use and move the ADDRESS of "user_buffer" to it
int 22h -- call the interupt to read in user data