Usually they give you a spray in the throat that will make the back of your throat a little numb, so that you don't retch. The sedation is usually Midazolam a medicine that does not let you go to sleep properly (so you do everything they say) but makes you forget everything between having the (intravenous) injection and what happens in the next hour after it.
Endoskopy is not bad.
the camera is about the diameter of your index finger and slides over your tongue and down the back of your throat. the worst bit is the first 10 seconds so if you get all panicky and feel choked then don't worry it won't last that long. it feels much easier once it has passed into your food pipe.
You'll be laid on your side on a bed with a thing in your mouth like a gumshield (protects your teeth, and stops you accidently biting down on the endoscope)
You WON'T be sick during the procedure. trust me, have seen dozens of them and never seen anyone sick. you might feel bloated cos of the air they blow into your stomach, and be burping alot through the procedure. don't worry though, everyone does!
The camera will automatically take pitcures of your food pipe (particulaly where it ends and stomach begins) and in your stomach, as well as the first ickle bit of intestine. they might also take a couple of small samples, but you can't feel them take these (your stomch doesn't 'feel' like skin does). they might then take one of these samples to test for a bug thats in some peoples stomachs, you'll get the results of that (probably via your GP) within a few days.
The whole thing only lasts about 4 minutes (honest!) and then if you have been sedated they'll want to keep an eye on you for at least an hour before your folks take you home. you'll be dopey/sleepy as hell after so in no state to do anything in particular.
hope that helps