Kids don’t sleep. The phrase “sleeping like a baby” is there just to torment you. Babies rarely sleep, unless they are being held or attached to a boob. Other than that, they’ll spend possibly their first two years waking every two hours. Yes, I said years. You will become a zombie whose sole purpose in life is to keep the kid(s) alive until bedtime. At such a time, you will (again) go to bed desperately hoping tonight will be the night that (s)he sleeps through the night. It won’t.
Pictures of kids with food all over their faces are never cute. Ever. Even though you think your cherub is adorable, no one else will. The older the child, the more horrendous it becomes.
Stop buying toys. The most entertainment your child will get from any toy is watching you impale yourself on it a hundred times, mostly in the middle of the night (see #1) when you’re trying to be quiet.
You will become that parent. That will be anything that annoyed you before you had kids and vowed you would never let your child do.
Young children are uncivilized little beasts with zero self-control who give you a glimpse of what it is like to live with a mental patient. Oddly enough, you will worship these illogical, egocentric time bombs.