It's not going to be that simple, as you don't know how much is in there already, oil levels have to be a set measurement (and weight of oil), so you need to start with the fork empty of any oil.

Would be looking at the PDF instructions of your fork (all major makes will be on their manufactures websites) to see how much, for how to service instructions, check out Youtube, as someone will (probably) have posted a video of how to on there.