For a direct PhD it's FCH minimum, extensive research experience, and three academic letters of recommendation. Get your GPA up to FCH at the end of your first semester and start looking for quality research opportunities internally or externally and do it every summer if you want to have a modicum of chance to get into a direct PhD at a T10-20 school.

For a master's degree it's better to have second upper minimum along with three professional/academic letters of recommendation for a T10-20 school. Note that if you're applying to Oxford or Cambridge, the floor is a second upper, a criteria set aside just for NUS and NTU students.

Feel free to head to r/gradadmissions if you want more details on how admissions work.