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!