What I find helpful when doing these problems is separating the original equation into two. Since the problem calls to find the oxidizing agent and reducing agent, its easier to see in 2 equation. Begin with creating 2 separate equation that include the same elements.
So, from Co2+ (aq) + HNO2 (aq)
NO (g) + Co3+ (aq)
Notice one part of the reactants contains Copper and on part o the products contains Copper, pair these up.
Co2+ (aq)
Co3+ (aq)
Now assign Oxidation states to the substances on both sides.(Hint: Oxidation state is equal to the charge of the molecule)
Once you know the oxidation state for both sides of this mini equation, decide whether it is losing electrons or gaining electrons.
(The gaining of electrons is reducing while the losing of electrons is oxidation)
Do the same with the other mini equation and decide which is causing the other to lose electrons and which is causing the other to gain.
Hope this helps. Sorry I'm new and find it hard to explain things online.