Aww I remember them well, such an amazing experience.
The 12 week scan is pretty quick generally, they measure bubs for dates and check how many babies your having and look if everything looks ok, but most of the checking for anomalies etc is done at 20 weeks.
Go with a full bladder but not too full else you'll be in agony when they're pressing on you, some places you have to have cash/change to pay for the photo's (I think Birmingham I had to have pound coins and buy a ticket from a machine), so it's worth checking how much they are (think my last ones in Coventry were about £7 each).
Some people suggest drinking a can of coke or eating chocolate before the scan to wake bubs up, my daughter waved/kicked/wiggled loads through my scans, my son never moved in any of them!
Enjoy- my first scan was one of the best experiences of my life, seeing my daughter (now 3) waving her little arms and legs and thinking "there's actuaaly a real person in my tummy"