Clothes - a loose fitting top and trousers or skirt are a good idea as you need to be able to pull clothes up and down easily. Also short or loose sleeves. I'm assuming you've already been told what tests you will be offered.
My hospital says full bladder but I've found that getting there with time to drink a little water but not huge quantities and not doing what I'd normally want to do before going into see them has been enough on each occasion, and I've had 3 scans this time - the first showed that I was nearly 2 weeks earlier than I thought so too early for nuchal fold test, the second was where they did my tests, and the third was part of the CVS test I had.
I had my first baby at 37 and my tests - like yours the combined test with bloods as well - came back as low risk, that sonographer said on the day that the nuchal fold looked normal. They wrote to me by post.
This time my results came back as high risk -the nuchal fold result was ok but the blood test wasn't plus my age is already high risk on its own, as I just turned 39 at the end of June. I opted for CVS as the quickest way of getting definite results (fortunately better news this time after a nailbiting weekend).
I found the NHS booklet on screening tests one of the most useful things I read, of course I didn't read it before I got back my dodgy results. Apart from anything else a lot of published books seem to have inaccuracies about important things like when you can have specific tests, whereas the NHS booklet is given to you free and details current practice.