I have an example "I love it here in NY. It's too good." I know that it should be "very good" or "so good" but I don't know why "too good" is wrong.
"Too good" is not wrong. It is a hyperbolic expression that means really really good. However, you would use it conversationally or in creative or informal writing. You should not use it in academic writing.
"Too good to be true" is an expression that means something shouldn't be trusted. It means that something is not as good as it seems and there must be hidden problems.