Answer
Nitrogen is oxidized
Hydrogen is reduced
Work Step by Step
N2 + 3H2 --> 2NH3
N2 and H2 are in their elemental form so they both have oxidation states of 0.
On the right we have NH3. Here Hydrogen is -1 so we have 3*-1 = -3. Since overall NH3 must have a charge of 0 then Nitrogen is +3
This means Nitrogen goes from 0 to +3
Hydrogen goes from 0 to -1
Nitrogen loses electrons (is oxidized)
Hydrogen gains electrons (is reduced)
