There are so many variables that can affect your experience. Whether or not you have eaten recently is a big one. A lot of people that try hydro for the first time on an empty stomach end up getting nauseus and possibly throwing up. Whether or not you are legitimately in pain is another biggie. If you are, well forget the high because they are painkillers and will do their job. If youre simply looking to have a good time, 15mg is a good dose for a beginner. I wouldnt eat immediately before taking them, but having a completely empty stomach isnt something i would recommend for a beginner, so eat about 4 hours beforehand. The amount of APAP in them can also affect you because acetominophen in large doses can definitely upset your stomach.
Take the 15mg, preferrably when you dont have to worry about anything or anyone else for a number of hours so your head is clear and dont have so many expectations. Just swallow them like a normal pill then do whatever you find to be fun that isnt physically demanding. Hell, i would watch a movie and literally just chill.
They will work, so take them, cop a suat on the couch and watch a movie for a couple hours. You will melt into the couch in about an hour and before you know it, you will be on movie 2 and still be feeling good.
Dont clutter your head with the "am i high" thoughts and just let them do what they do. Good luck.