Does the President have to declassify something BEFORE releasing the information?

As chief executive, he can decide to declassify material as he sees fit. Usually there are procedures to ensure that the information released does not damage sources or negatively interferes with other operations, etc. In the most recent case, Trump shared information gathered by another country and that violates the usual agreement that the source dictates how that information may be shared.

So Trump did not violate any law, he is correct in saying it was his right. However, it is potentially very damaging to the usefulness of the intel, the relationship with Israel (they may not want to share intel with us as freely) and could get the person working for the Israeli intel agency killed (it may have come from an inside agent who has now been compromised).

When Bush 43 outed Valarie Plame as a CIA operative, he simply said the words. He then said it was a secret again.
Until now, that was the height of hypocrisy.

The president can do what he/she wants with regards to national security. What Trump did was reckless and moronic, but not illegal (If anyone else had done it it would have been illegal though).

I do think he should be impeached, not for this, but for violating the Emoluments clause.