Well, literally speaking, unless you are disable in every imaginable way, you are still useful. If nowhere else, then helping your mom.
To me, being useful means that you have skills and talents and you utilize them. That you utilize yourself, really. (not my way to do things but seems to work for majority..)
But why do you want to be useful anyway? What happens when you're useless? What if you are doing something that gives you great joy and at the same time is totally and utterly useless(for others and yourself alike)? Have you been told that you're lazy?