NP-completeness: Reduce to or reduce from?
To show a new problem $X$ is hard, you reduce from a problem of known hardness to $X$. For example, you can show $X$ is hard by reducing 3-SAT to $X$.

The general idea is that if you had an efficient algorithm for solving $X$, you could also solve instances of 3-SAT efficiently. This is so because you can exhibit a polynomial time transformation from a 3-SAT instance to an instance of $X$. Given an instance of 3-SAT, you transform it into an instance of $X$, apply the efficient algorithm for $X$, and thus have solved the 3-SAT instance.