Wikipedia has a page on the Greek Alphabet, showing the conversion between its letters and the ones used in the Latin alphabet. This will help you decipher the text, but not the meaning. Below is a screenshot of the conversion table which can be found on the linked page:
Last time I went on a road trip in Greece (2008) I don't remember reading that many road signs in Latin alphabet, but I might obviously be wrong. In my opinion the best tip I can give you is to learn the Greek names of the place you will be heading to, and their representation in Greek alphabet. To this purpose, here is a Wikipedia page with a list of the largest cities in Greece. Following on the individual page for each city you will find their modern Greek names. Just to give you a few examples of you should expect to read (signs are spelt in UPPERCASE):
- English Name - Greek Name - Greek Spelling
- Athens - Athina - ΑΘΗΝΑ
- Thessaloniki - Thessaloniki - ΘΕΣΣΑΛΟΝΙΚΗ
- Patras - Patra - ΠΑΤΡΑ
- Sparta - Sparti - ΣΠΑΡΤΗ
You should also probably learn keywords such as
no parking, and any other phrase you might typically find on a road sign.
For more general tips on driving in Greece, this website provides a set of helpful guidelines on the topic.