Hi, i am a Singaporean and intend to visit Tibet for less than 15 days, do i need a visa.

A China visa is not required for you to visit Mainland China (including Tibet) for less than 15 days, but you need to apply for a Tibet Entry Permit, otherwise, you can't enter Tibet, please contact a travel agency in China to get the Tibet Entry Permit for you.

What documents are needed for the application of the Tibet Entry Permit?
Do i have to pay to get the Tibet Entry Permit and is the processing done immediately?

Kathleen Tan, a list of tour group to the Tibet is required, you don't need to provide documents, maybe the travel agency will ask you to join its group tour and then obtain the Tibet Entry Permit for you. You can first enter Mainland China, such as Xian, Beijing, Shanghai, ask the travel agency to send the Tibet Entry Permit to you when you arrive, it is free to get the Tibet Entry Permit for the travel agency, but you need to pay service fee for the travel agency if you don't join its group tour.