I have two of them. That is an incredibly difficult question with an incredibly long answer, but I can be long winded.
Expect seeing the woman you love transform into the most amazing creature on earth. Expect seeing her belly grow with your child, a part of you inside there. Expect to go through nights where she thinks the baby is coming, and it isn't. Expect that when it does come, that it will be the most amazing and terrifying experience you could ever imagine. Expect seeing the woman you love in excruciating pain. Expect your child to come out looking like an alien, but loving them more than your life itself. Expect nights with absolutely no sleep, like none, zero, no minutes, hours or seconds. Expect diapers filled with strange goo for the first few weeks and wondering how an earthling could actually expel such nasty stuff. Expect to see your kiddo go from a lump of baby to a person in a short period of time. Expect to see some of your flaws in them, and see how beautiful they can be from a parent's eyes. Expect your family to be over a LOT more than before and also expect to go from wanting privacy from them to wanting them around every minute. Expect fights with your wife and expect a significant work life balance issue early on. That's about it for now.