So guys the thing is I'm 18 years old, obviously this is the time where I'm supposed to be having as much fun as possible. To me this means not being tied to one girl. I don't want a monogomous relationship, but I have no idea how to sort out a multiple relationship where I would have a few girls that I would go out with, that would all know about eachother without me having to lie to them. Can anybody help me out on this?

I want to give you a tip.. tell the girls you approach from the beginning that you are not so boyfriend material. Somewhere during talking when things become more sexual. It will save you from problems arising later. I needed over 9 months to have the balls telling my GF that I want an open relationship/MLTR. Damn 9 months! Be straight at the beginning, lying is not necessary. If a girl is not okay with it, you don't want her anyways (control freak/jealous type/whatever).
There's an approach that you should have a "queen" in MLTRs (your hb8/9+) and tell her that, and several other relationships on a sexual base only.
Well, just be honest... lying is so stressful