If you aren't sure about your strengths, ask some of your peers or professors what they see as your best qualities. Refer to any written feedback or references you've received in the past from peers or managers. Before the interview, prepare a list of your skills that best match those the employer is seeking. This list can include education or training, soft skills, hard skills, or past work experiences.
During the interview, discuss attributes that will best qualify you for the specific job and set you apart from the other candidates.

"My greatest strength is my problem-solving skills. I can also work to tight deadlines under pressure. For example, I was once asked to complete a project that fell through the cracks. My supervisor discovered the mistake two hours before the deadline. It was an important client and a potentially award-winning project. Not only did the project have to go out on time, but it had to be perfect. I hunkered down and got busy. We were able to submit the project for review on time and it was recognized by the Institute of Industrial Engineers for an environmental sustainability award."