Question

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

Answer 1

It means that you are exactly what you say you are.
Our mental attitudes determines the thoughts we have and our thoughts determine the words we say and as a man thinks in his heart so he is. This means that if you think it is impossible for you to fail, then you will discover that your efforts will be helped by situations and circumstances around you and you will succeed because you have created a positive energy around you by your words. On the other hand, if you think you can not succeed, then you will not, because your words will create negative situations around you that will ensure that you fail. So, at the end you get exactly what you say you will get.

Answer 2

It means that the way to look at a situation will affect how you act. If you go into a quiz or test at school saying to yourself "I can't do this" then you are more likely to not do well because of your mind set. On the other hand, if you go into the same test saying "I can do this!" then you are more likely to succeed because you believe in yourself. It's all about your mental mind set.
Hope this helps! Please let me know if I missed something :)