It is not a simple process to join the UN. First, all five members of the UN Security Council must approve your application. Those five countries are China, France, Russia, United Kingdom, and the United States. Then at least four more members of the rotating, non-permanent members of the Security Council must also approve. The application is then brought to the floor of the General Assembly, where two thirds of members most vote affirmatively.
The most recent country to join the UN is South Sudan, which joined in 2011. Sudan was effectively split into two pieces, with the southern piece gaining its independence. This ended a five-year civil war. Sudan (in the north) is primarily Arab who practice Islam while South Sudan is mostly comprised of black citizens who practice Christianity and indigenous religions.