It could be GBL, which is dosed in mls; if it's GHB then we have no way of advising you on the dose safely as sekio said. Start very low, and wait 2 hours before redosing - redosing more quickly is a surefire way to knock yourself out. What dose did the guy who gave it to you suggest? In future, try to find out the concentration. Something like GHB/GBL has a steep dose:response curve and you could very easily find yourself unconscious. It's really dangerous to have it dosed in a few drops, as people may well consume a dangerous overdose thinking they need a lot more..