If you mean the general GRE, then you already have the necessary math background -- the highest math on the general GRE is precalculus. Though, you should still do practice quantitative reasoning, and study vocabulary for verbal and the way they ask questions in the verbal section.
If you mean the math GRE, you definitely want to wait until you've had upper-level courses (abstract algebra, real analysis, etc.), so I'd wait until my senior year. Although half of it is calculus, the other half tests a lot of these advanced topics, and a wide variety of them at that.