Sometimes you can select a topic from a list and sometimes you can chose any topic under the sun - think about what they are trying to gauge.
Pick a topic that
you are familiar with, but not technical in nature
allows you to demonstrate learning/growth in your personal life
you are passionate about or can interview/discuss in some level of detail
Focus on maintaining eye contact, calmness in your voice, some slight tone variation, confidently respond to answers and don't rush through what you are saying - its often a matter of slowing your mind rather than talking too fast
Hope this helps