what is the best starting point to prepare for graduate school if you are not well equipped?
You might want to take a few courses (at first)
as a special student, a non-degree seeking student.

However, the standards for admission are not less than regular admission invariably.

This is especially true at Ivy League universities 

See my comments regarding being admitted as a
graduate special student at Harvard University I posted on this forum:

https://academia.stackexchange.com/a/61120/123306