One Hanja every Korean learner should know 正 (한자) | Korean FAQ
I usually don't recommend that beginners learn Hanja (한자), as its benefits come mostly later on through vocabulary and making new words.
However, there are some exceptions. One of those is 正 (read as "정"). This character is useful even as a beginner, especially if you're going to be ordering food in a Korean restaurant. And fortunately it's very easy to read and fast to write with only 5 strokes.