"I told you so" is a set expression in English that means "I was right, and you were wrong." You say "I told you so" when:

1. you disagreed with someone about a choice
2. the other person didn't listen to your advice
3. the other person's choice turns out to be wrong

"I told you so" sounds a little mean, so if you're really trying to be polite, you should avoid saying it. People also introduce it like this:

I hate to say it, but I told you so.

I hate to say 'I told you so', but...