We all have things we can improve on, so do not be afraid to share them. The interviewer will likely take note of these things and may even provide you with additional training or mentor opportunities in these areas to help you excel.
The interviewer wants to hear that your weaknesses are not critical for the job to which you are applying. Think of a couple of things you could improve upon that are not necessary for the job, and openly share them with the interviewer.
"I would say that my biggest weakness is in the lack of work experience that I do have; since I am newer to my career. With that said, I am a sponge when it comes to learning new skills and am confident that the time you take to train me will be swift and simple."