As an expert on terrorism for the United Nations, what advice would you give to world leaders to help end terrorism?

First, the goal of ending terrorism is very unlikely to be achievable. Since a key feature of good goal setting is to think realistically, it might be best to aim to reduce deaths from terrorism, something that is a more realistic aim.
Next, it would be important to emphasize the need for ongoing study of different terrorist groups and what motivates them and the efficacy of different types of anti-terrorist interventions. This should be done by international panels of social scientists rather than politicians who use fear of terrorism as a way to suppress civil liberties and secure votes. It is important to remember that in an average year many more people die from everyday threats like car accidents than from terrorism and to avoid creating the sort of media hype which only encourages people to engage in terrorist attacks for attention.
In terms of demographics, terrorism seems to appeal primarily to young men who are fueled by feelings of isolation, resentment, and lack of purpose. To reduce the likelihood of their actually committing terrorist acts, community programs are needed to give them other options in their lives.
Another major motivation for terrorism is a feeling of helplessness in the face of injustice. Oppression of minority groups, especially in areas with strong tribal or ethnic loyalties, can lead to outbreaks of violence. Addressing poverty and injustice will also minimize the number of people who resort to terrorism.
Obviously, international cooperation on security is also important, but in the long run, just as preventing diseases is better than curing them after the fact, so too is eliminating the root causes of terrorism more effective in the long run than investing exclusively in trying to catch would-be terrorists.