I was very nervous before my scan too, but actually ended up enjoying it.
The 20 week scan is quite a detailed check of various parts of your baby. I don't remember the full list, but the sonographer will check things like their spine, their head, their heart, their kidneys, etc, to make sure everything is ok. I found it totally amazing to see things like my baby's spine. It was beautiful
Sometimes they can't see a particular body part because of baby's position, so you may have to pop back so they can see it. I had to do this, but they did it the same day.
I'm not sure how long the scan took, but it was longer than normal because of the all the things to check. I'd guess maybe half an hour? Something like that anyway.