Well, I know what your first word was. It was waaahhhhhhh!
(I don't see why this shouldn't count as a word. It's a sound made as a means of communication, and it certainly gets its message across.)
Yes, I think wah! is a word. I'm not so sure about hello.
Hello was a sound one of my daughters used to make when she was baby-babbling. We made a game of repeating it to each other, and soon when anyone went up to her pram and said hello, the six-week-old baby inside would say hello right back.
It gave some little old ladies quite a shock, I can tell you.
Ah, people said, but she doesn't know what it means. Fair enough: after all, I'm not sure what hello means, myself.
Not easy, is it?
As for the word that babies say first when they begin to build a vocabulary (oh, and how much easier everything is once they have) then I'm afraid Dadda is the commonest, followed by Mamma. Then, according to one survey, it's dog, cat, more, baby, ball, duck, teddy, milk, Gran, again.
More and again are not the names of things, and first words quite often aren't. My brother's first word was neath (his toy car had vanished under the sofa) and other reported first words include cool, bye bye, oh dear, okay, garbage out! and let go!
What I'd really like to know is, do the babies who say duck first turn out to be animal-lovers, and the babies who say cookie turn out to love food?
Now, there's a nice fifty-year long research project for someone.
Word To Use Today: first. This word comes from the Old English fyrest and is related to the Old Saxon furist, the Old Norse fyrstr and the German Fürst, prince, one who is first in rank.