What is the first element to start filling a d sublevel?

The answer would be Scandium, with an atomic number of 21. This is a chemical element with symbol Sc and atomic number 21. A silvery-white metal d-block element, it has archaeologically been ordered as a rare earth element, organized with yttrium and the lanthanides.

Scandium is the first element to fill the d sublevel. The d orbital are occupied transition metals and Scandium is a transition metal with atomic number 21 thus has the following electron configuration (Ar) 3d^1 4s^2. From this electronic configuration scandium has only one electron in the d subshell.