The best way to prepare for the NLE is often just to take practice tests. From year to year, the format of the NLE is quite similar, and you will begin to see patterns as to what kind of questions are asked.
If you are taking the NGE for the first time, it requires very basic knowledge of Greek history and language. Start out by learning the Greek alphabet. Once you familiarize yourself with the letters, it will become much easier to read and understand Greek, which is actually similar to Latin in many ways. Most (if not all) of the content on the NGE is included in the syllabi.