@Gold, I had one a few years ago, and I didn't have sedation as I was going home on the back of DH's motorbike...!
I did have to fast beforehand, so it might be worth checking whether your appointment is for a consultation rather than the actual procedure.
PPs have already given you a lot of useful information, so I won't duplicate what they said; the procedure did make me gag a bit, but concentrating on my breathing worked, and the tube is quite small and doesn't impede your breathing at all.
The local anaesthetic spray to the back of your throat is fairly effective, although mine tasted like the worst Pina Colada I've EVER had - I think it was supposed to be banana flavoured. I think.
It was all over very quickly, and at no point was it painful, just uncomfortable.
The only thing I will add is to go for a pee shortly before the procedure and wear a maxipad or one of Tena's finest, because if you are like me (slightly weak of sphincter) and you do find you gag a bit, you might leak slightly. I knew nothing about this and had a full bladder and no protection when I was called in for mine - I left a bit of a puddle on the couch 
But honestly, I remember that as being the worst part of it. So it can't have been too bad!
You'll be fine
