1mg is 3iu, so a 6mg vial is 18iu. So, from there, the amount of water you use will determine how much you draw into the insulin syringe. If you mix it with 9ml of water, every ml would be 2iu. Just take your time and think it out. Be sure to store your GH in the fridge before and after mixing!
As for dosing, Serostim is one of the best GH's out there. If you haven't been taking growth, you might want to start out with less, like 2iu and see how you respond, before increasing