Well, you certainly want to be safe, so having and maintaining a comfortable hold and firm grip around his waist (and maybe even with your thumbs and fingers gripping his belt or waistband) is necessary unless there might be some other provision such as a strap or handgrips, but there likely will not be either of those available. And depending upon his size and your arm length, you might decide to just reach all the way around and clasp your hands together in front of him ... and there is nothing either right or wrong about any of that. Riding two-up is a circumstantially-cozy situation where everyone understands it must be that way simply for the sake of safety.
Then, and depending ever so greatly upon the character of the man taking you for a ride, there is the simple matter of wearing a sports bra or something bulky (or however you might occasionally deal with that) to avoid the kind of body contact that could be distracting (large breasts pressing against one's back) if you might decide you feel most comfortable and safe while pulling yourself up tight as you hang on. But if he is a decent, safe and considerate rider, you really should not have to hang on quite that tightly.
On another note: As best you can, mentally prepare now for *not* leaning away from a turn to either side. Rather, just stay in your sitting position directly behind him and "go with the flow" as the bike leans one way or the other while turning ... for if he is making a turn and you lean away from it to try to remain vertical to the ground, you easily could cause him some control problems.
If you can, freely ask him questions before you actually go for a ride ... and if he is an experienced rider and aware of what it is like to be on the back, he will likely understand and be happy to put you at ease.
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