Start your GRE prep early, ideally the spring/summer before you intend to apply; your normal coursework plus the grad school applications are a lot of work, so don’t add studying for the GRE to your burden. Try to take the GRE over the summer or early in the fall semester for applications that are due in December/January. 

The GRE is split into three content sections: Verbal Reasoning, Quantitative Reasoning, and Analytical Writing.