Spectral Leakage: Number of Samples
The DFT of a discrete-time sinusoid will only have components at the sinusoid's frequency if there are an integer number of periods inside the window. I.e., you must figure out the period $M$ of the sinusoid and choose the number of points $N$ as an integer multiple of $M$ ($N=kM$). I'm sure you can figure out the period of the given sinusoid.

Also have a look at this answer to a related question.