You have to add things to the half-equation in order to make it balance completely.
All you are allowed to add are:
electrons, water, hydrogen ions(unless the reaction is being done under alkaline conditions - in which case, you can add hydroxide ions instead).
First balance the number of species on both sides of the equations. Then determine the change in oxidation state of nitrogen. If it's increasing, then there's loss of electrons, so you should add that much number of electrons on right hand side of the equation. If the oxidation state decreases, then there is gain of electrons, so you should write that much electrons at left hand side of the equation.