The electrons fill energy levels from the lowest available energy level, based of three typical rules.
1) Follow Aufbau rule: fill from lower to higher energy levels (1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p)
2) Pauly exclusion principle: two electrons can not have the same four quantum numbers
3) Hund rule: if the orbitals have the same energy, the electrons must go to different orbitals before two occupies the same orbital.