I think for most people it just clicks. Someone in here said it's not hard. I disagree (though I may have read the "it's not hard" in a different sense of "it's not hard"). It's definitely hard work. I now have a great respect for any stay at home parent. A child is so much work. And there are so many levels of hard. For instance, we looked forward to when our little one could walk. We thought it would make things so much easier. No that was just a different level of hard.
But our kid is so much fun. Before he came I didn't have any experience with kids, at all. So I was a little scared that I wouldn't know what to do (hence the books). But it just came naturally. Changing diapers, playing (either "roughhousing" or "nicely"), feeding. It just happened. And for those other moments where I didn't know what to do, well I had my wife to lean on.
I would echo don't listen to others (* this does not include your pediatrician and mother's obgyn) and don't compare your little person to others.
My only piece of advice (except the one I just gave) is find a way to really cherish the moments. I always thought "they grow so fast" was bs. Well two years plus in and I still look at our kid and wonder where the last two years went. It has been a blast but I now look at a little adult sometimes and wish I had a little more time. Work has definitely been put in its place over the course of the last year as I realized this.