If $G$ is a group of order $p^2$ with $p$ a prime number then $G \simeq C_{p^2}$ or $G \simeq C_p \times C_p$
If order of group is $p^2$, where $p$ is prime, how can you deduce $G$ is isomorphic to $C_{p^2}$ or $C_p \times C_p$?