I've always been an atheist, but I can remember at the age of 10 saying I was a Christian becuase I felt pressured from my Christian peers. What I did was wrong. I think it is unhealthy to deny who you are and what you believe.
If someone does ask, I usually say "I'm not religious" and leave it at that. I tend to avoid debating - arguing religion solves nothing and usually ends up offending people and their belief systems, and even a healthy religious debate will go on for eternity with nothing gained. You can't argue logic and proof with someone who bases their beliefs on a lack of logic or proof (faith).
I'd tell your family and stand firm on your beliefs if they ask. Being an atheist is nothing to be ashamed of. Quite the contrary actually.