When should I take the GRE and how do I prepare?

[F1225]

Hi there!

I am a second year undergraduate students and planning to take GRE.
When should I apply it?
What should I prepare for this test?
Are there any sample questions of the test available?

Thanks a lot for helping.

 

[Septim]

Yes there are past exam questions available on the website http://grephysics.net/ans/ you can check it but it is limited to four tests.