I wouldn't feel comfortable attributing this cliche/poster thingy/etc. to anybody, but you can find attributions if you look around:
"It's not that `today is the first day of the rest of my life,' but that now is all there is of my life."
Today is the first day of the rest of your life.
"Today is the first day of the rest of your life!"
"Breakfast of Champions"
If today is the first day of the rest of your life, what the hell was yesterday?
"Also, get PERSONAL POWER over your POTENTIAL and LEARN HOW TO ASSERT IT!! Grok yourself fully during QUALITY TIME!! Dare to be great! Remember: TODAY IS THE FIRST DAY OF THE REST OF YOUR LIFE!!"
~Kermit Busganglion, The Hand You're Dealt