How do I calculate the oxidation state of Nitogen in NH4NO3?

You divide NH4NO3 into two parts:
+ Part 1: NH4+ it contains the electric charge of +1, so the sum of oxidation state will be +1
 Remember that H is always +1, Oxygen is always -2
Hence, in NH4+  : call the oxidation state of Nitrogen is x: x +4 = 1 => x = -3
+ Part 2: NO3- containing the electric charge of -1, so the sum of oxidation state will be -1
Hence, in NO3- call the oxidation state of nitrogen is y : y + (-2)*3= -1 => y = +5
In conclusion, in NH4NO3, Nitrogen have two different oxidation state : - 3 and + 5 in two different atoms N

The sum of the oxidation states of all the atoms in the molecule is zero.
Oxidation state of oxygen = -2
Oxidation state of hydrogen = +1
Let the oxidation state of nitrogen be x.

2x + 4 * 1 + 3 * (-2) = 0
2x + 4 - 6 = 0
2x - 2 = 0
x = 1