if this is your first time, I wouldn't start with more than 1ml. Remember, you can always increase but you can't take it back once you've swallowed it. The first time I did it, I took 1ml to start, felt fantastic, about 1.5 hours later, took one more 1ml.
we always make sure that out of the group of people, there is one 'sober' person (somebody NOT taking the G) who keeps track of how much each of us has had and keeps the G. Wanting to redose is common, and thus making it very easy to overdose because your inhibitions are lowered (ah, fuck it, ill be fine, gimme another...or...shit, did I have 1 ml or 2? ahh Ill take another...=trip to hospital).
BE CAREFUL. Google "how to dose GHB" and please read a bit about it before you embark on your experience. GHB has many many clauses, so it's important to know exactly what you're getting yourself into. If you wake up and your ass hurts, you might have had a little too much. (jus keeeeeeeding)