Which orbital in a subshell gets filled first?
The notation [-1,0,1] is given to address the sub shells.The energies of all these orbitals (unless you apply a magnetic field) are identical, therefore the filling is completely arbitrary. That means any of the shells can be filled first. But after 1 shell is filled the next electron goes to any one of the remaining shells. And the next goes to the other one. This is known as the Pauli's Exclusion law. Electrons with like spins are filled first and electrons with the opposite spin are filled next (note: 1 shell can share 2 electrons). This choice of spin is also complete arbitrary.