Your best bet is to send a short, to-the-point but friendly AW message asking if the girl is free for an incall at time x, for duration y etc. Many girls won't bother replying to anything less specific, probably because there are so many time wasters around.
Your inexperience is neither here nor there to most girls while arranging the booking, but feel free to mention it when you turn up, and any good escort will be understanding.
Most importantly, be sure to pick a well-reviewed girl on here, rather than just any random girl who takes your fancy on AW. This can be a really varied hobby, the best girls are amazing, but the worst can be terrible, and you really don't want to risk a bad experience for your first time. Good luck!