for that u have to read a chapter called co-ordination chemistry.

few things are imp

1) sum of the oxidation states of a neutral complex is 0

2) sum of the oxidation state of a charged complex is equal to its charge it could be -ve or +ve.

3)read abt ligands like neutal ligand like NH₃ ,H₂O,NO,CO etc they have zero total oxidation state

4)negative ligands, they have total oxidation state equal to their total charge.

eg CL-,Br-

 now your example

[Cr(C₂O₄)₃]^3-

Its a negative complex so total oxidation state is equal to -3 thats the total charge.