What does Say I can or say I can't and you're right either way mean?

"Say I can" means that you're telling me I can do something. "Say I can't" means you're telling me I cannot do it. This quote is talking about the power of Words - if you tell someone that they can do something, many times, they end up working hard to accomplish their goal. The opposite is also true - if you tell someone that they can't do something, often they will give up and not even try. So "You're right either way" means that if you tell me something often enough, I'm going to believe it and it will become true.