depends on what kind of pain you have... is it post surgury? is it chronic? nerve pain?
a low starting point, generally, would be anywhere from 5mg every 8 hours to 5mg every 6 hours
however, depending on the severity of the pain a pain doctor could start with more. there really is no way to determine how much you need unless you see a doctor who specializes in pain management... not to mention we have no idea what kind of pain you have.
and again we cant tell you how much you need to get high without knowing your tolerance. however, for a beginner, id say 10-15 mg would get you a good buzz, dont overdue it or you could throw up. you can always do more so start low.