I don't know how to stop a leaking tap, do you?

I'm at this age, where people would call me a grown up. But yet there are so many things I'm clueless about, just wondering, are there other people out there that lack so many apparently practical skills- like changing a tyre, a light bulb, declaring taxes, finding an appropriate investment to invest in.

Also, if you could learn one practical thing, what would it be?