It's guaranteed to happen at every interview – as you near the end, your interviewer will look up and ask “so…do you have any questions for me?”. You must be aware that replying “no” comes across as disinterested, so it's always best to have a few questions prepared in advance.
A good starting point is to ask “What does a typical day in this role look like?” Obviously no two days are the same, but it shows you're keen to know more about the job and, in turn, get a feel for whether you really want it.
You could also ask “What's the workplace culture like?” Workplace culture is incredibly important, and will dictate how happy you are in the role. You could ask the interviewer what they enjoy about working there, which also shows an interest in them.
Asking about opportunities for career development is another smart move. It shows your potential employer you're ambitious and want to contribute to the organisation.
You might also want to get an idea of the bigger picture, by asking “What are the company's long-term goals and ambitions?” Thinking beyond this role and this department demonstrates an interest in how the organisation works as a whole, where your particular cog would fit in the wider workings.
Finally, you definitely want to ask what the next step in the interview process will be. Not only does this show an eagerness to progress, but it gives you a good idea of time frames and what to expect next.
Having your questions clear in your mind will reduce the stress when the moment comes, and show potential employers how much you want the job.
What does a typical day look like?
What is the workplace culture like?
Are there opportunities for career development?
What are the company's goals?
What is the next step?