I have taken GRE twice. One somewhat mundane comment: please ensure proper rest before taking the exam. The first time I took it, hadn’t slept enough. The environment was so calm and quiet that I fell asleep during the English section of the test – if I remember correctly. Although I woke up before the test had ended, many questions just had to be guessed. Books can be helpful: Princeton Review, Barron’s. However, an online class from, as an example, Princeton Review, can certainly be even more beneficial.
I think Junior year would actually be slightly early to start thinking about the GRE. Maybe plan on taking it the Summer before Senior year. This way, you won’t experience the pressure of college and you will be able to fully focus on taking the exam. As far as I know, the GRE test results are valid for 5 years, so I think that is always good to have in hand in case you want to go to grad school at some point.