こんにちは。
"how (best) to pass the time" 又は "what to do with the time" は両方とも「時間をどうやって過ごせば良いのか」を意味します。
「人と会っても、共通の話題がなくて、どうやって時間を過ごして良いのか分からないことです」=>
"When you meet someone but you don't have anything in common to talk about, it's hard to know how best to pass the time."
共通の話題がない = you don't have anything in common to talk about
共通 = in common
it's hard to 〜 = 〜は難しいです
ご参考になれば幸いです。