The oxidation number of a free element is always 0.

The oxidation number of a monatomic ion equals the charge of the ion.

The oxidation number of [Math Processing Error] is +1, but it is -1 in when combined with less electronegative elements.

The oxidation number of [Math Processing Error] in compounds is usually -2, but it is -1 in peroxides.

The oxidation number of a Group 1 element in a compound is +1.

The oxidation number of a Group 2 element in a compound is +2.

The oxidation number of a Group 17 element in a binary compound is -1.

The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.

The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

For finding oxidation state of any element first we must know the oxidation state of oxygen like oxygen oxidation state is (-2)
let suppose the element be x .we have to know the oxidation state of carbon so,
for ex- CO2=x+(-2)×2=0. = +4
I hope you understand