for me, it was actually while my wife was still pregnant. I asked someone "I don't think I'm ready?!?!!!" and he said, "you're not supposed to be. Just remember that when the baby is born, you will be 'born' into fatherhood, too."
kinda just helped me realize that I don't have to be a perfect dad to be a good dad and have fun along the way...
The best piece of advice my dad ever gave me, about being a dad myself, was to never be afraid to let your kids see your emotions. There's a stigma and assumption that men can't be emotional by nature and have to keep up a facade of being calm and cool at all times.
I let my kids see my emotions. They know when I'm angry. They see me when I cry while I'm deeply worried about family and loved ones. They see me being humble when I need to apologize about something. And when I'm happy, they see that as well.
As long as these emotions aren't taken to extremes, i think it does children good, especially sons, to know that they don't have to be ashamed of their emotions and they can be free to feel what they feel.
Down the road, being comfortable with their emotions will help open the lines of communication down the road.