When I first has my scan the other day I was so nervous. I didn't find much info online about what to expect so hopefully I'll share as much as I can with what I learnt.
Drink lots of liquid, BUT don't do this hours and hours before otherwise you'll be bursting before you even get there. I drank about 750ml of bottled water an hour and a half in sips before the actual scan and that was enough!
Think of some questions to ask the person doing the scan whilst you are on the bed. I wish I had asked more questions now, just general things like what was she measuring, etc. It will reassure you and you won't be kicking yourself afterwards for not asking things.
Wear something comfy that unbuttons low, he/she will pour a jelly like substance on your belly and push the scanner onto your tummy firmly. If you feel a bit uncomfortable down there a while afterwards, I believe that is quite common so don't panic.
Don't forget to bring all of your notes, they need all of this! Without it I'm sure they turn you away. They are so used to doing this so it'll be clockwork I'm sure when you get there, will be thinking of you hoping you feel less nervous x
Oh and I nearly forgot, if you want photos you normally have to pay anything from £1.50 - £3 for them but it is donatino based so pay what you can x