Most error coins do not fetch the real high prices that you see on the internet..keep that in mind.
But if it is a real error coin and not a post-production damaged coin (which is what most people find when they think they have an error coin) is still not worth much unless its a very obvious and big error. So take it to a coin dealer and get their thoughts. If they offer you some decent money for it, take it right there because it will be impossible for you to find the buyer who will give you the top max dollar for it. The dealer knows who that one or two guys in the country are who will buy it, but you don't and it may take you a real long time to find that guy. Error coins are a pretty small market overall.