Representatives of each member country meet in the General Assembly, this is the closest thing to a world parliament.
Each country has a single vote. Decisions that are voted for become resolutions.
Major decisions about things like who can join the UN need the support of two-thirds of the Assembly.
Less important decisions need the support of over half the Assembly.
The Assembly's decisions are not laws. Governments follow resolutions because they represent the world's opinion on major international issues.