how to protect myself if someone know my social security number ?
and what can he actually do with it ??
Talk to your local credit bureau and the local police. They both have professionals who can assist you protect yourself and determine if there is any real risk
If someone knows your SSN you'd best be protected by purchasing some form of identity mugging protection and monitoring your credit report, if you have a credit card allot of CC companies offer this service. If someone knows your SSN they can do allot from applying for credit cards to purchasing things on credit similar to houses, cars etc. You can find out anything you want about someone if they have that information and know how to use it.