Writing equation of reaction in stoichiometry of redox titrations
See the following steps:


We know what $\ce{KMnO4}$ will do by reading the question:


  Calculate the volume of […] $\ce{KMnO4}$ solution required to oxidize completely […] oxalic acid.

We need the structure of oxalic acid. (This is knowledge required to answer the question, no way around.)


Structure of oxalic acid. Image taken from Wikipedia, where a full list of authors is available.
Determine oxidation numbers of all the atoms involved. Shortcut: hydrogen has $\mathrm{+I}$, oxygen has $\mathrm{-II}$ and carbon has $\mathrm{+III}$.
Determine what can be oxidised. Only carbon really can.
Determine what it can be oxidised to. The final product must have $\ce{C^{IV}}$, which is most common as carbon dioxide or carbonate.
Solve the redox equation using either $\ce{CO2}$ or $\ce{CO3^2-}$. Both will work.