GRE is a reputed examination and for this reason one has give more efforts when it comes to score a decent score. Most of the aspirants opt for US and others in European countries. One can find many good material available on internet platform.
Before starting any exam, I recommend you to try to give a diagnostic test. from the results, you can come to know where you stand and what areas you should focus more. As I am from an engineering background, It was pretty easy for me to crack the Quant section. I had a nightmare about Verbal section. I analyzed that GRE has 70% reading comprehension stuffs, So I gave more time practicing more RC passages from GRE recommended books and free material from Internet. In parallel, my vocabulary was also weak, so I planned to learn around 30 words per day. Choose any one material and jot it down and memorize it. Start from easy level to the toughest level.
For the writing section, One needs to understand the pattern of how to write it. One can find good material on youtube such as NOTEFULL( Though it is for TOEFL, but useful too), etc.
Practice this for 15 days to a month and give another diagnostic test. If one feels that he/she has not performed well then go for online or classroom coaching.
Wishing you best of luck for the preparation.