You must be 13 years old to sign up on Facebook. Go to www.facebook.com. Sign up. Don't forget which email address you are using and what your password is.
If you are a minor child, do not post your address or your phone number online. Don't accept "friend" requests from people that you don't personally know. And don't forget to give your password to your mom and dad, if you are a minor. It's for your safety.