To get to the United Nations, you can take one of the following buses: 8, 18, F, V or Z, direction "Nations". From Place des Nations, walk uphill along "Avenue de la Paix" (Peace Avenue). The "Palais des Nations" public entry is next to a tourism school.
Geneva is the home of the European headquarters of the United Nations. There are tours offered in any of the organization's 15 official languages. I took one of the tours in English. The explanations were not that interesting, but taking one of the guided tours was the only way to visit the complex of buildings. So, I do recommend taking one tour. All you have to do is leave your passport at the entry and pay a moderate price. Unfortunately, the gardens cannot be visited.