First, learn to forgive yourself for your failings as a parent. Because you will be tested mentally, beyond anything you have experienced as a young single guy or gal. you will end up losing your shit at times and you will feel sorry/shame afterwards. And then it will most likely happen again, many times, and then you will, if you're smart, learn your new triggers and start to become wiser.

When the kid/s are young (and if you are an active participant, as opposed to a 1980s style parent), you will be pushed physically, more tired than you ever thought possible, and on top of that, you will have spousal issues too as you navigate a new landscape. (If you are an old school parent, you will be busy at work or the bar while your spouse does all the work and you will be oblvious to the building resentment and biterness building up until it blows you on you one day in mount Vesuvious fashion and you'll wonder where it came from).

Life is no longer a stress-free series of lovely meals and wild sex, it can still be those things, but with the added fear that your child might die if something or be kidnapped. A new dimension of terror exists, and it won't ever go away, then if you have grandkids, you'll worry about them too.

Don't compare your kids to other people's kids. Comparison is the thief of joy, and this applies to kids, too.

Finally, I will say this: I think it's the ultimate learning experience, and each kid is different. I think it's like each child is a different lock and your job, as a parent (and a teacher, if you are one, like @cryotosensei) is to find the right 'key' and approach. What works for other parents may or may not work for you.