This type of question always makes me nervous! But it is important to have a well-prepared answer.
Think about 1-2 achievements or accomplishments from your professional life, and 1-2 from your personal life.
What were the events? Why are they particularly important to you? What was the situation in which they occurred? Be prepared to give the background to the achievement, a description of what you achieved, and why this particular achievement is important to you. What did you learn from it? How has it made you better at your job?
It's good to have one example from work, and one from your personal life prepared. It can show that you are balanced in your life (and it is easier to talk about the two things for 5 minutes each than about 1 thing for 10 minutes).
Practice your answer out loud. You don't want it to sound memorized, but it will help you remember what you want to say, as sometimes when we are nervous our brains can go a bit blank!
The site www.businessenglishpod.com has really good resources for interviews. You might find some useful tips there.
I hope this helps! Good luck with your interview!