The UN Department of Peacekeeping Operations looks after UN peacekeeping interventions.
Do these operations require unanimous support, majority support or something else?
According to Wikipedia, the missions are approved by the UN Security Council after a peace treaty has been negotiated:
Once a peace treaty has been negotiated, the parties involved might ask the United Nations for a peacekeeping force to oversee various elements of the agreed upon plan. This is often done because a group controlled by the United Nations is less likely to follow the interests of any one party, since it itself is controlled by many groups, namely the 15-member Security Council and the intentionally diverse United Nations Secretariat.
If the Security Council approves the creation of a mission, then the Department of Peacekeeping Operations begins planning for the necessary elements. At this point, the senior leadership team is selected. The department will then seek contributions from member nations. Since the UN has no standing force or supplies, it must form ad hoc coalitions for every task undertaken. Doing so results in both the possibility of failure to form a suitable force, and a general slowdown in procurement once the operation is in the field.