Take a look at sprintf() (man sprintf
What you will need to do is create a string variable to hold the command, and a string to hold the user's name. Something like this:
// Get the user name
scanf("Enter a user name: %s", user_name);
// Now, put 'du -ks /home/' followed by the user's name
// in the system_command variable.
sprintf(system_command, "du -ks /home/%s", user_name);
// Execute the command
EDIT: The scanf above should be split; it should be a printf followed by a scanf, but that's probably not that important. You already had the code to read the user's name anyway...