Hey there,
Usually, as part of the initial prompt, the interviewer will also ask the first question (e.g. "how would you approach structuring this problem?"). However, if this is not the case, I wouldn't just "wait" for the interviewer to ask - if the first question is not 100% clear to you, at the end I would clarify - "OK, so I think I understand the problem. Should I start by laying out my approach to tackling this problem?"
Hope this helps - don't worry too much about this, the interviewer will be experienced and will know how to guide you so I doubt this will be an issue!