Most important thing, is to chose your companion well. Based on your needs I agree with avoiding backpage randoms, and stick with a well reviewed agency or Indy girl. It probably will cost you more, and I have no idea what your budget it (tho I can surmise based on your initial comments about looking for a 30min session, and comments about rates being non negotiable that budget may be an important consideration for you). Even with this in mind, if your goal is to gain some more sexual experience and even some "coaching" to augment your skills in the sack, you may do better with a higher end Indy girl, or well reviewed experienced agency girl. I'd be honest with the so in what you are looking for. Many will gladly "show you the ropes" I'm sure. If you are worried about finishing quickly, let her know and ask to start slow, maybe enjoying a bit of a sensual back rub first. Also if you have the means, if you choose to book 60min, even if you finish quick for the first time, there will be lots of time left for #2.
As the others have said, be clean, respectful, punctual and settle the business part as soon as you enter.