All foreigners including Singaporeans need to possess a set of travel permits in order to enter/visit Tibet.
The first permit that all foreigners need to obtain is the Tibet Travel Permit which is only issued by the Tibet Toursim Bureau. The Tibet Travel Permit has to be produced during domestic flight check-in to Lhasa.
The next permit that you may need, depending on which areas you plan to visit, is the Alien Travel Permit which is issued by the Public Security Bureau. You will need this permit if you plan to travel outside of the Lhasa-Shigatse area. You will need this permit if you plan to visit Everest Base Camp which is outside Lhasa-Shigatse area.
There is a third permit called the Military Permit which is only needed for visiting remote and sensitive places in Tibet like Mount Kailash or Chamdo. Hence majority of tourist visiting Tibet only need to possess the first two permits (Tibet Travel Permit and Alien Travel Permit).
All the above permits can only be applied through a travel agency who will handle all the necessary paper work when you sign up a tour with them.
Further permits are just 1 thing you need to visit Tibet. You also need a tour guide, private vehicle and driver. All these can only be arranged through a travel agency and have to be settled before you step into Tibet.
However if you are a Chinese citizen, you can visit Tibet by yourself without above restrictions.