Just think of the acronym STAR when answering. Situation/Task - describe the situation or task Action - describe the action taken Result - describe the result. It is best to give an example that has a positive outcome but even those with a negative outcome are welcome if you follow with a statement about what you learned or what you would do differently in the next situation.
This was difficult mainly because it was the very first question they threw at me. I hadn't even applied to the job directly, so I didn't have a good idea of what they were looking for and that made it difficult for me to describe myself in a way that showed how I could meet their needs.