If you have a router in place, log into the Router and look at the Status tab (Linksys) or another tab that has the configuration on it. Your public IP address will NOT start with 192 or 168.
You can also try websites like this one, but they might not be 100% reliable.
Why would you want to know your public IP? BTW, for most ISP, this address is not static, it can change over time. If you are trying to connect home from outside, I suggest you check one of those free DNS service provider like DynDNS.