I first said "I love you" to my cat...
As for that of a person, it was some random girl I don't remember in third grade. Naturally, I didn't know what the hell those three words actually meant; rather, those words were said to convey a desperate and strange need to have "girlfriend" in the third grade. Still trying to figure that one out - a "girlfriend" in the third grade?

As for the first time it was truthfull said:

To none other than my current girlfriend, of whom I wil be sharing a very happy one year within the next two months. She said it to me first, around five months ago, and I mulled the notion and feeling through my head for several days before I returned the magical phrase. So simple, those three words, yet so very powerful and captivating. However cliche, it was beneath a moonlit sky full of stars atop a hill overlooking the valley. A gentle breeze, ourselves wrapped in a blanket, etc... your classic romantic night.

It just seemed.. right to have it be that night that she knew I loved her back.