Your best strategy for navigating the UN site is to know which agency is involved in the issue.
The organization chart will help you to identify agencies and their relationships.
The WHO World Health Organization is the primary UN agency for global health. Search for "policy" on the home page, or "topic AND policy."
Some very worthwhile materials for analyzing public policy do not come from books or peer-reviewed journals. Learn to mine the sites of governments and IGOs (inter-governmental organizations); their documents are primary sources.
You might also find good analysis and research on issues through NGOs (non-governmental organizations) and think tanks or research institutes. This type of research requires good critical thinking skills--evaluate the reports you find for any bias, advocacy, etc. and take that into account when you present your arguments.
IGO Research Guide is a searchable database of information and links to IGO sites.