Just one lesson - I'm not even sure that is possible. Patience was a huge lesson, I thought I didn't have any before having the twins. Sleep was also a big lesson, such as how much you can survive without before collapsing. But really the biggest lesson was probably instinct.
There is no manual on being a parent or having kids. While you are pregnant, especially with more than one your first time out, fears overcome you. How will I know what they need? How to care for them? Will I be able to tell them apart? How will I feed them both at the same time?
But once they are here in your arms, instinct just kicks in. Yes there are moments when you have no clue what to do, but a lot more times that you think, you will know what needs to be done. The best part of having children is that you grow with them. When they are born, you only have to worry about their VERY basic needs. You don't have to teach them anything the first few weeks, allow them to teach you. And then as they grow, you will grow and the rest is... well not in the bag, but not what I feared at all.